A multicatchment analysis of headwater and downstream temperature effects from contemporary forest harvesting

Stream temperature is a key physical water‐quality parameter, controlling many biological, chemical, and physical processes in aquatic ecosystems. Maintenance of cool stream temperatures during summer is critical for high‐quality aquatic habitat. As such, transmission of warm water from small, nonfish‐bearing headwater streams after forest harvesting could cause warming in downstream fish‐bearing stream reaches with negative consequences. In this study, we evaluate (a) the effects of contemporary forest management practices on stream temperature in small, headwater streams, (b) the transmission of thermal signals from headwater reaches after harvesting to downstream fish‐bearing reaches, and (c) the relative role of lithology and forest management practices in influencing differential thermal responses in both the headwater and downstream reaches. We measured summer stream temperatures both preharvest and postharvest at 29 sites—12 upstream sites (4 reference, 8 harvested) and 17 downstream sites (5 reference, 12 harvested)—across 3 paired watershed studies in western Oregon. The 7‐day moving average of daily maximum stream temperature (T_7DAYMAX) was greater during the postharvest period relative to the preharvest period at 7 of the 8 harvested upstream sites. Although the T_7DAYMAX was generally warmer in the downstream direction at most of the stream reaches during both the preharvest and postharvest period, there was no evidence for additional downstream warming related to the harvesting activity. Rather, the T_7DAYMAX cooled rapidly as stream water flowed into forested reaches ~370–1,420 m downstream of harvested areas. Finally, the magnitude of effects of contemporary forest management practices on stream temperature increased with the proportion of catchment underlain by more resistant lithology at both the headwater and downstream sites, reducing the potential for the cooling influence of groundwater.     

The effect of transient storage on nitrate uptake lengths in streams: an inter‐site comparison

Surface water–groundwater interaction in the hyporheic zone may enhance biogeochemical cycling in streams, and it has been hypothesized that streams exchanging more water with the hyporheic zone should have more rapid nitrate utilization. We used simultaneous conservative solute and nitrate addition tracer tests to measure transient storage (which includes hyporheic exchange and in‐stream storage) and the rate of nitrate uptake along three reaches within the Red Canyon Creek watershed, Wyoming. We calibrated a one‐dimensional transport model, incorporating transient storage (OTIS‐P), to the conservative solute breakthrough curves and used the results to determine the degree of transient storage in each reach. The nitrate uptake length was quantified from the exponential decrease in nitrate concentration with distance during the tracer tests. Nitrate uptake along the most downstream reach of Red Canyon Creek was rapid (turnover time K^?1_c = 32 min), compared with nitrate uptake reported in other studies (K^?1_c = 12 to 551 min), but other sites within the watershed showed little nitrate retention or loss. The uptake length S_w‐NO^?₃ for the most downstream reach was 500 m and the mass transfer coefficient V_f‐NO^?₃ was 6·3 m min^?1. Results from 15 other nitrate‐addition tracer tests were used to create a regression model relating transient storage and measures of stream flow to nitrate uptake length. The model, which includes specific discharge and transient storage area, explains almost half the variability in nitrate uptake length (adjusted R² = 0·44) and is most effective for comparing sites with very different stream characteristics. Although large differences in specific discharge and storage zone area explain inter‐site differences in nitrate uptake, other unmeasured variables, such as available organic carbon and microbial community composition, are likely important for predicting differences in nitrate uptake between sites with similar specific discharge rates and storage zone areas, such as when making intra‐site comparisons. Copyright © 2007 John Wiley & Sons, Ltd.     

Wood distribution in neotropical forested headwater streams of La Selva,Costa Rica

Surveys of wood along 30 forested headwater stream reaches in La Selva Biological Station in north‐eastern Costa Rica represent the first systematic data reported on wood loads in neotropical streams. For streams with drainage areas of 0·1–8·5 km² and gradients of 0·2–8%, wood load ranged from 3 to 34·7 m³ wood/100 m channel and 41–612 m³ wood/ha channel. These values are within the range reported for temperate streams. The variables wood diameter/flow depth, stream power, the presence of backflooding, and channel width/depth are consistently selected as significant predictors by statistical models for wood load. These variables explain half to two‐thirds of the variability in wood load. These results, along with the spatial distribution of wood with respect to the thalweg, suggest that transport processes exert a greater influence on wood loads than recruitment processes. Wood appears to be more geomorphically effective in altering bed elevations in gravel‐bed reaches than in reaches with coarser or finer substrate. Copyright © 2009 John Wiley & Sons, Ltd.     

Field studies on factors affecting very fine and ultra fine particulate organic matter deposition in low‐gradient sand‐bed streams

The knowledge on particle deposition in streams is mainly based on investigations in mountain streams. No data exist from low‐gradient sand‐bed streams that largely differ in the morphological and hydraulic factors proposed to affect deposition. To identify physical control on particle deposition in low‐gradient streams, we assessed deposition of very fine and ultra fine organic particulate matter in 18 sand‐bed stream reaches. We added particles derived from lake sediment and assessed the mean transport distance S_P and the deposition velocity v_dep. Additionally, reach hydraulics were estimated by injections of a conservative solute tracer (NaCl). Among the low‐gradient streams, particle deposition kinetics were variable but similar to deposition in mountain streams. S_P was solely related to the flow velocity. This relation was confirmed when comprising published data on deposition of fine organic particles. An association between particle deposition and transient storage factors was insignificant. We found significance of the transient storage to S_P only for repeated measures within a single reach, when flow velocity and benthic conditions were nearly constant. Measured v_dep/v_fall ratios were much larger than unity in most reaches. Evidence from this relation suggests that the vertical transport of very fine and ultra fine organic particulate matter through the water column was caused mainly by vertical mixing. Copyright © 2006 John Wiley & Sons, Ltd.     

Seasonal dynamics and exports of elements from a first‐order stream to a large inland lake in Michigan

Headwater streams are critical components of drainage systems, directly connecting terrestrial and downstream aquatic ecosystems. The amount of water in a stream can alter hydrologic connectivity between the stream and surrounding landscape and is ultimately an important driver of what constituents headwater streams transport. There is a shortage of studies that explore concentration–discharge (C‐Q) relationships in headwater systems, especially forested watersheds, where the hydrological and ecological processes that control the processing and export of solutes can be directly investigated. We sought to identify the temporal dynamics and spatial patterns of stream chemistry at three points along a forested headwater stream in Northern Michigan and utilize C‐Q relationships to explore transport dynamics and potential sources of solutes in the stream. Along the stream, surface flow was seasonal in the main stem, and perennial flow was spatially discontinuous for all but the lowest reaches. Spring snowmelt was the dominant hydrological event in the year with peak flows an order of magnitude larger at the mouth and upper reaches than annual mean discharge. All three C‐Q shapes (positive, negative, and flat) were observed at all locations along the stream, with a higher proportion of the analytes showing significant relationships at the mouth than at the mid or upper flumes. At the mouth, positive (flushing) C‐Q shapes were observed for dissolved organic carbon and total suspended solids, whereas negative (dilution) C‐Q shapes were observed for most cations (Na⁺, Mg²⁺, Ca²⁺) and biologically cycled anions (NO₃^?, PO₄^3?, SO₄^2?). Most analytes displayed significant C‐Q relationships at the mouth, indicating that discharge is a significant driving factor controlling stream chemistry. However, the importance of discharge appeared to decrease moving upstream to the headwaters where more localized or temporally dynamic factors may become more important controls on stream solute patterns.     

Soil pipe flow tracer experiments: 2. Application of a streamflow transient storage zone model

Soil pipes are important subsurface flow pathways in many soil erosion phenomena. However, limited research has been performed on quantifying and characterizing their flow and transport characteristics. The objectives of this research were to determine the applicability of a streamflow model with transient storage in deriving flow and transport characteristics of soil pipes. Tracer data from pulse inputs were collected in four different soil pipes after a fluorescein dye was injected in the upstream end of each soil pipe network in three branches (west, middle, and east) of a main catchment and a back catchment in Goodwin Creek Experimental Watershed in Mississippi. Multiple sampling stations were positioned along each soil pipe network. The transient storage zone model OTIS‐P was executed inversely to estimate transport parameters by soil pipe reach such as the soil pipe cross‐sectional area (A), soil storage zone cross‐sectional area (A_s), and exchange rate between the soil pipe and the soil storage zone (α_s). Model convergence was achieved, and simulated breakthrough curves of the reaches were in good agreement with actual tracer data for eight of the nine reaches of the three branches of the Main Catchment and five of the seven reaches of the Back Catchment soil pipe. Simulation parameters for the soil pipe networks were similar to the range of values reported for flow and transport characteristics commonly observed in streams. Inversely, estimated soil pipe flow velocities were higher with increased tortuosity, which led to a smaller cross‐sectional areas predicted for the soil pipe flowpaths, while other parameters were not sensitive to tortuosity. In general, application of One‐Dimensional Transport with Inflow and Storage‐P to this unique soil pipe condition suggested larger transient storage (A_s and α_s) compared with most stream systems. This was hypothesized to be because of relatively higher ratio of the wetted perimeter to flow area in the soil pipe, the hydraulic roughness of the soil pipe, potential retention in collapsed portions of the pipe, and interaction with smaller preferential flow systems. Copyright © 2015 John Wiley & Sons, Ltd.     

Relation between flow,surface‐layer armoring and sediment transport in gravel‐bed rivers

This study investigates trends in bed surface and substrate grain sizes in relation to reach‐scale hydraulics using data from more than 100 gravel‐bed stream reaches in Colorado and Utah. Collocated measurements of surface and substrate sediment, bankfull channel geometry and channel slope are used to examine relations between reach‐average shear stress and bed sediment grain size. Slopes at the study sites range from 0·0003 to 0·07; bankfull depths range from 0·2 to 5 m and bankfull widths range from 2 to 200 m. The data show that there is much less variation in the median grain size of the substrate, D_50s, than there is in the median grain size of the surface, D₅₀; the ratio of D₅₀ to D_50s thus decreases from about four in headwater reaches with high shear stress to less than two in downstream reaches with low shear stress. Similar trends are observed in an independent data set obtained from measurements in gravel‐bed streams in Idaho. A conceptual quantitative model is developed on the basis of these observations to track differences in bed load transport through an idealized stream system. The results of the transport model suggest that downstream trends in total bed load flux may vary appreciably, depending on the assumed relation between surface and substrate grain sizes. Copyright © 2007 John Wiley & Sons, Ltd.     

Large wood in central Appalachian headwater streams: controls on and potential changes to wood loads from infestation of hemlock woolly adelgid

Large wood (LW) is an important component of forested headwater streams. The character of LW loads reflects a balance between adjacent valley processes that deliver LW to the channel (herein recruitment processes) and stream channel processes that either retain or transport LW through the reach (herein retention processes). In the central Appalachian Mountains, USA, LW characteristics in headwater streams located in eastern hemlocks (Tsuga candensis) forests are expected to change because of infestation of hemlock woolly adelgid (Adelges tsugae, HWA), an exotic, invasive insect. We examined LW characteristics in 24 headwater streams ranging from un‐infested to severe infestation, as determined by hemlock canopy health. The objectives of this work were to: (i) quantify wood loads; (ii) assess the relative importance of valley recruitment and in‐stream retention mechanisms in controlling reach‐scale wood loads; and (iii) assess if there was a detectable influence of HWA on LW loads. We hypothesized that LW loads would be similar to other forested streams in eastern USA and dominated by recruitment processes. In addition, higher LW loads would correspond with advanced HWA infestation. Mean wood frequency was 38 pieces/100 m ± 17 (standard deviation); mean wood volume was 3.69 m³/100 m ± 2.76. In general, LW load characteristics were influenced by both recruitment and retention parameters; jam (accumulations ≥ 3 pieces) characteristics were dominated by retention parameters. Results suggest that adjacent stand basal area influences LW loads and once LW is recruited to the channel, streams lack sufficient hydraulic driving forces, despite having lower resistance structures, to transport LW out of the reach. Sites in moderate decline had higher proportions of short (1–2 m and 1–4 m) and very long (>10 m) LW with higher frequency of jams that were low in volume. We present a hypothesized conceptual model of expected changes to LW loads associated with HWA infestation and hemlock mortality. Copyright © 2015 John Wiley & Sons, Ltd.     

Hydraulic characteristics of a small Coastal Plain stream of the southeastern United States: effects of hydrology and season

Various physical and biological properties affect solute transport patterns in streams. We measured hydraulic characteristics of Payne Creek, a low‐gradient upper Coastal Plain stream, using tracer experiments and parameter estimation with OTIS‐P (one‐dimensional transport with inflow and storage with parameter optimization). The primary objective of this study was to estimate the effects of varying discharge, season, and litter accumulation on hydraulic parameters. Channel area A ranged from 0·081 to 0·371 m² and transient storage area A_s ranged from 0·027 to 0·111 m². Dispersion D ranged from 1·5 to 11·1 m² min⁻¹ and exchange coefficient α ranged from 0·009 to 0·038 min⁻¹. Channel area and dispersion were positively correlated to discharge Q, whereas storage area and exchange coefficient were not. Relative storage size A_s/A ranged from 0·17 to 0·59, and was higher during fall than other seasons under a similar Q. The fraction of median travel time due to transient storage ranged from 8·8 to 34·5% and was significantly correlated with Q through a negative power function. Both metrics indicated that transient storage was a significant component affecting solute transport in Payne Creek, especially during the fall. Comparison between the measured channel area A_c and A suggested that surface storage was dominant in Payne Creek. During fall, accumulation of leaf litter resulted in larger A and A_s and lower velocity and D than during other seasons with similar discharge. Seasonal changes in discharge and organic matter accumulation, and dynamic channel morphology affected the magnitude of transient storage and overall hydraulic characteristics of Payne Creek. Copyright © 2005 John Wiley & Sons, Ltd.     

Bankfull Discharge Recurrence Interval in Gravel-bed Rivers

Bankfull discharge was identified in some 30 gravel-bed rivers representing in total c. 40 gauging stations. The catchment sizes cary from 4km² to nearly 2700km². Bankfull discharge value increases with basin size. In the case of gravel-bed rivers developed on an impermeable substratum, the following equation emerges: Q_b=0·087 A^1·044. Bankfull discharge recurrence interval was determined by fitting maximum annual floods (T_a) into Gumbel's distribution and then using the partial duration series (T_p) in this same distribution. Recurrence interval is below 0·7 years (T_p) for small pebble-bed rivers developed on an impermeable substratum; it reaches 1·1 to 1·5 years when the catchment size of these rivers exceeds 250km². Rivers incised in the soft schists of the Famenne show larger channel capacity at bankfull stage, a small width/depth ratio and thus higher recurrence intervals (1·4–5·3 years with T_a and 1–4·4 years with T_p). Baseflow-dominated gravel-bed streams and sandy or silty rivers experience less frequent bankfull discharges, with a recurrence interval higher than 2 or even 3 years (T_p). © 1997 John Wiley & Sons, Ltd.     

Patterns of bedload entrainment and transport in forested headwater streams of the Columbia Mountains,Canada

We monitor bedload transport and water discharge at six stations in two forested headwater streams of the Columbia Mountains, Canada. The nested monitoring network is designed to examine the effects of channel bed texture, and the influence of alluvial (i.e. step pools and riffle pools) and semialluvial morphologies (i.e. boulder cascades and forced step pools) on bedload entrainment and transport. Results indicate that dynamics of bedload entrainment are influenced by differences in flow resistance attributable to morphology. Scaled fractional analysis shows that in reaches with high form resistance most bedload transport occurs in partial mobility fashion relative to the available bed material, while calibers finer than 16 mm attain full mobility during bankfull flows. Equal mobility transport for a wider range of grain sizes is achieved in reaches exhibiting reduced form resistance. Our findings confirm that the Shields value for mobilization of the median surface grain size depends on channel gradient and relative submergence; however, we also find that these relations vary considerably for cobble and gravel bed channels due to proportionality between dimensionless shear stress and grain size. Exponents of bedload rating curves across sites correlate most with the D_90s of the mobile bed, however, where grain effects are controlled (i.e. along individual streams), differences in form resistance across morphologies exert a primary control on bedload transport dynamics. Application of empirical formulae developed for use in steep alpine channels present variable success in predicting transport rates in forested snowmelt streams. Formulae that explicitly account for reductions in mobile bed area and high morphological resistance associated with woody debris provide the best approximation to observed empirical data. Copyright © 2014 John Wiley & Sons, Ltd.     

Macrophyte‐driven transient storage and phosphorus uptake in a western Wisconsin stream

Investigations of phosphorus cycling and transport in streams lend insight into potential mechanisms of nutrient sequestration and can help mitigate human impacts. In this study, we examined the relationship between transient storage and phosphorus uptake in a cold‐water stream in western Wisconsin. Hydrological characteristics, nutrient spiralling metrics, macrophyte biomass, and geomorphological properties were quantified in 7 reaches of Spring Coulee Creek using injections of a conservative tracer alone or with added PO₄^3?. Fraction of median travel time due to transient storage (F_med²⁰⁰) was correlated with macrophyte biomass (r = .794, p = .033), and PO₄^3? uptake velocity was correlated with F_med²⁰⁰ (r = .756, p = .049). Stepwise linear regression was used to build models for transient storage and uptake velocity. Macrophyte biomass, stream bed slope, and riffle to pool ratio accounted for 99.6% of the variation in transient storage (p < .001). Transient storage, canopy cover, and slope accounted for 98.0% of the variation in uptake velocity (p = .002). This study shows that transient storage, primarily resulting from macrophyte beds, can be a significant factor regulating phosphorus uptake in stream ecosystems.     

Stream nitrate uptake and transient storage over a gradient of geomorphic complexity,north‐central Colorado,USA

The understanding of nutrient uptake in streams is impeded by a limited understanding of how geomorphic setting and flow regime interact with biogeochemical processing. This study investigated these interactions as they relate to transient storage and nitrate uptake in small agricultural and urban streams. Sites were selected across a gradient of channel conditions and management modifications and included three 180‐m long geomorphically distinct reaches on each of two streams in north‐central Colorado. The agricultural stream has been subject to historically variable cattle‐grazing practices, and the urban stream exhibits various levels of stabilisation and planform alteration. Reach‐scale geomorphic complexity was characterised using highly detailed surveys of channel morphology, substrate, hydraulics and habitat units. Breakthrough‐curve modelling of conservative bromide (Br^?) and nonconservative nitrate (NO₃^?) tracer injections characterised transient storage and nitrate uptake along each reach. Longitudinal roughness and flow depth were positively associated with transient storage, which was related to nitrate uptake, thus underscoring the importance of geomorphic influences on stream biogeochemical processes. In addition, changes in geomorphic characteristics due to temporal discharge variation led to complex responses in nitrate uptake. Copyright © 2011 John Wiley & Sons, Ltd.     

Hydrologic and biogeochemical controls on the spatial and temporal patterns of nitrogen and phosphorus in the Kuparuk River,arctic Alaska

Nitrogen (N) and phosphorus (P) dynamics in the Kuparuk River in arctic Alaska were characterized in a 3‐year study using routine samples near the mouth of the river at the Arctic Ocean, synoptic whole‐river surveys, and temporally intense sampling during storms in three headwater basins. The Lower Kuparuk River has low nitrate concentrations (mean [NO₃]‐N] = 17 µg l^?1 ± 1·6 SE) and dissolved inorganic N (DIN, mean [N] = 31 µg l^?1 ± 1·2 SE) compared with rivers in more temperate environments. Organic forms constituted on average 90% of the N exported to the Arctic Ocean, and high ratios of dissolved organic N (DON) to total dissolved N (TDN) concentrations (mean 0·92) likely result from waterlogged soils formed by reduced infiltration due to permafrost and low hydrologic gradients. Annual export of TDN, DON, and particulate N averaged 52 kg km^?2, 48 kg km^?2, and 4·1 kg km^?2 respectively. During snowmelt, the high volume of runoff typically results in the highest nutrient loads of the year, although high discharge during summer storms can result in substantial nutrient loading over short periods of time. Differences in seasonal flow regime (snowmelt versus rain) and storm‐driven variation in discharge appear to be more important for determining nutrient concentrations than is the spatial variation in processes along the transect from headwaters towards the ocean. Both the temporal variation in nitrate:DIN ratios of headwater streams and the spatial variation in nitrate:DIN between larger sub‐basins and smaller headwater catchments is likely controlled by shifts in nitrification and soil anoxia. Copyright © 2008 John Wiley & Sons, Ltd.     

Characterizing stream temperature hysteresis in forested headwater streams

Stream temperature (T_s) is a key water quality parameter that controls several biological, ecological, and chemical processes in aquatic systems. In forested headwaters, exchanges of energy across air-water-streambed interfaces may influence T_s regimes, especially during storm events as the sources of runoff change over space and time. Analysis of the hysteretic behaviour of T_s during storm events may provide insights into rainfall-runoff responses, but such relationships have not been thoroughly investigated. As such, our objectives were to (a) quantify the variability of stream temperature hysteresis across seasons in different sub-regions and (b) investigate the relationship between the hysteretic response and catchment characteristics. T_s hysteresis during storm events was assessed based on the hysteresis index (HI), which describes the directionality of hysteresis loops, and the temperature response index (TRI), which indicates whether T_s increased or decreased during a storm event. We analysed T_s data from 10 forested headwater reaches in two sub-regions (McGarvey and West Fork Tectah) in Northern California. We also performed a clustering analysis to examine the relationship amongst HI, TRI, topographic metrics, and meteorological characteristics of the study areas. Overall, the hysteretic behaviour of T_s varied across seasons—the greatest HI occurred during spring and summer. Interestingly, in the McGarvey streams the variability in T_s hysteresis co-varied strongly with topographic metrics (i.e., upslope accumulative area, average channel slope, topographic wetness index). Comparatively, in West Fork Tectah the variability of T_s hysteresis co-varied most strongly with meteorological metrics (i.e., antecedent rainfall events, solar radiation, and air temperature). Variables such as the gradient between stream and air temperatures, slope, and wetted width were significant for both sub-regional hysteretic patterns. We posit that the drivers of T_s response during storms are likely dependent on catchment physiographic characteristics. Our study also illustrated the potential utility of stream temperature as a tracer for improving the understanding of hydrologic connectivity and shifts in the dominant runoff contributions to streamflow during storm events.     

Landscape influences on aluminium and dissolved organic carbon in streams draining the Hubbard Brook valley,New Hampshire,USA

Concentrations of both aluminium (Al) and dissolved organic carbon (DOC) in stream waters are likely to be regulated by factors that influence water flowpaths and residence times, and by the nature of the soil horizons through which waters flow. In order to investigate landscape‐scale spatial patterns in streamwater Al and DOC, we sampled seven streams draining the Hubbard Brook valley in central New Hampshire. We observed considerable variation in stream chemistry both within and between headwater watersheds. Across the valley, concentrations of total monomeric aluminium (Al_m) ranged from below detection limits (<0·7 µmol l⁻¹) to 22·3 µmol l⁻¹. In general, concentrations of Al_m decreased as pH increased downslope. There was a strong relationship between organic monomeric aluminium (Al_o) and DOC concentrations (R² = 0·92). We observed the highest Al_m concentrations in: (i) a watershed characterized by a steep narrow drainage basin and shallow soils and (ii) a watershed characterized by exceptionally deep forest floor soils and high concentrations of DOC. Forest floor depth and drainage area together explained much of the variation in ln Al_m (R² = 0·79; N = 45) and ln DOC (R² = 0·87; N = 45). Linear regression models were moderately successful in predicting ln Al_m and ln DOC in streams that were not included in model building. However, when back‐transformed, predicted DOC concentrations were as much as 72% adrift from observed DOC concentrations and Al_m concentrations were up to 51% off. This geographic approach to modelling Al and DOC is useful for general prediction, but for more detailed predictions, process‐level biogeochemical models are required. Copyright © 2005 John Wiley & Sons, Ltd.     

Carbon isotope fractionation of dissolved inorganic carbon (DIC) due to outgassing of carbon dioxide from a headwater stream

The stable isotopic composition of dissolved inorganic carbon (δ¹³C‐DIC) was investigated as a potential tracer of streamflow generation processes at the Sleepers River Research Watershed, Vermont, USA. Downstream sampling showed δ¹³C‐DIC increased between 3–5‰ from the stream source to the outlet weir approximately 0·5 km downstream, concomitant with increasing pH and decreasing PCO₂. An increase in δ¹³C‐DIC of 2·4 ± 0·1‰ per log unit decrease of excess PCO₂ (stream PCO₂ normalized to atmospheric PCO₂) was observed from downstream transect data collected during snowmelt. Isotopic fractionation of DIC due to CO₂ outgassing rather than exchange with atmospheric CO₂ may be the primary cause of increased δ¹³C‐DIC values downstream when PCO₂ of surface freshwater exceeds twice the atmospheric CO₂ concentration. Although CO₂ outgassing caused a general increase in stream δ¹³C‐DIC values, points of localized groundwater seepage into the stream were identified by decreases in δ¹³C‐DIC and increases in DIC concentration of the stream water superimposed upon the general downstream trend. In addition, comparison between snowmelt, early spring and summer seasons showed that DIC is flushed from shallow groundwater flowpaths during snowmelt and is replaced by a greater proportion of DIC derived from soil CO₂ during the early spring growing season. Thus, in spite of effects from CO₂ outgassing, δ¹³C of DIC can be a useful indicator of groundwater additions to headwater streams and a tracer of carbon dynamics in catchments. Copyright © 2007 John Wiley & Sons, Ltd.     

Influences of sudden changes in discharge and physical stream characteristics on transient storage and nitrate uptake in an urban stream

Changes in the physical structure of urban streams can occur abruptly due to flashy high‐flow events and subsequently alter stream processes, including transient storage and nitrate uptake. We examined temporal variability in transient storage and nitrate uptake by exploring the effects of altered physical characteristics resulting from a single high‐flow event in three reaches of Spring Creek, an urban stream in Fort Collins, Colorado, USA. Study reaches of varying geomorphic and hydraulic characteristics were chosen to represent distinct geomorphic settings in terms of substrate size, sinuosity, bed slope, and degree of rehabilitation and structural controls. We performed detailed physical characterizations and multiple nutrient injections of Br^? and NO₃^? to estimate transient storage and nitrate uptake in each reach. A comparison of pre‐flood and post‐flood data indicates that transient storage and nitrate uptake are highly context specific and mediated by interactions between geomorphic setting and flood discharge. In the two reaches that showed significant post‐flood increases in transient storage (250% to 350% increases in F_med²⁰⁰), the pool‐riffle reach exhibited a significant increase in uptake velocity, while the channelized reach did not. In contrast, transient storage decreased post‐flood in the third reach containing hydraulic structures. These complex responses likely reflect reach‐specific differences in hyporheic versus in‐channel storage. This study shows that repeat injections are necessary to describe nutrient dynamics because transient storage and nitrate uptake can be highly variable over time (showing changes on the order of 100%) due to variation in discharge and geomorphically influential flow events. Copyright © 2014 John Wiley & Sons, Ltd.     

The Carbonate Radical (HCO3·/CO3–·) as a Reactive Intermediate in Water Chemistry: Kinetics and Modelling

Results from kinetic laboratory studies of reactions of the carbonate radical anion (CO₃^–·) with aromatic compounds in aqueous solution at T = 298 K are presented. Data were obtained in using a laser photolysis laser long-path absorption (LP-LPLA) apparatus which was designed for direct time-resolved studies of radical reactions. For the reactions of CO₃^–· with hydroquinone dimethyl ether (2), methyl anisole (3), benzene (4), p-xylene (5), toluene (6), chlorobenzene (7), nitrobenzene (8), and benzonitrile (9), rate coefficients of k₂ = (3.0 ± 0.6)·10⁷ M^–1 s^–1, k₃ = (9.7 ± 1.7)·10⁵ M^–1 s^–1, k₄ = (3.2 ± 0.7)·10⁵ M^–1 s^–1, k₅ = (3.8 ± 0.9)·10⁴ M^–1 s^–1, k₆ = (6.8 ± 2.3)·10⁴ M^–1 s^–1, k₇ = (2.7 ± 0.6)·10⁵ M^–1 s^–1, k₈ = (1.4 ± 0.5)·10⁴ M^–1 s^–1, and k₉ < 1.3·10² M^–1 s^–1 were obtained. In further studies the effect of temperature on the reactions (2), (4), and (5) has been studied. The kinetic data obtained for the reaction of the carbonate radical anion with aromatic compounds were compared to the corresponding reactions of the hydroxyl radical. Finally, these kinetic data were used within a simple model system to investigate the implications of carbonate radical anion kinetics within water treatment processes. It is shown that the degradation of organic pollutants in ^·OH-radical based water treatment may proceed via the CO₃^–·/HCO_3· radical under certain conditions.     

The influence of riparian vegetation on near‐bank turbulence: a flume experiment

Measurements from a fixed‐bed, Froude‐scaled hydraulic model of a stream in northeastern Vermont demonstrate the importance of forested riparian vegetation effects on near‐bank turbulence during overbank flows. Sections of the prototype stream, a tributary to Sleepers River, have increased in channel width within the last 40 years in response to passive reforestation of its riparian zone. Previous research found that reaches of small streams with forested riparian zones are commonly wider than adjacent reaches with non‐forested, or grassy, vegetation; however, driving mechanisms for this morphologic difference are not fully explained. Flume experiments were performed with a 1:5 scale, simplified model of half a channel and its floodplain, mimicking the typical non‐forested channel size. Two types of riparian vegetation were placed on the constructed floodplain: non‐forested, with synthetic grass carpeting; and forested, where rigid, randomly distributed, wooden dowels were added. Three‐dimensional velocities were measured with an acoustic Doppler velocimeter at 41 locations within the channel and floodplain at near‐bed and 0·6‐depth elevations. Observations of velocity components and calculations of turbulent kinetic energy (TKE), Reynolds shear stress and boundary shear stress showed significant differences between forested and non‐forested runs. Generally, forested runs exhibited a narrow band of high turbulence between the floodplain and main channel, where TKE was roughly two times greater than TKE in non‐forested runs. Compared to non‐forested runs, the hydraulic characteristics of forested runs appear to create an environment with higher erosion potential. Given that sediment entrainment and transport can be amplified in flows with high turbulence intensity and given that mature forested stream reaches are wider than comparable non‐forested reaches, our results demonstrated a possible driving mechanism for channel widening during overbank flow events in stream reaches with recently reforested riparian zones. Copyright © 2007 John Wiley & Sons, Ltd.