Mapping the spatial and temporal distributions of woody debris in streams of the Greater Yellowstone Ecosystem, USA

The objectives of this study were: (1) to document spatial and temporal distributions of large woody debris (LWD) at watershed scales and investigate some of the controlling processes; and (2) to judge the potential for mapping LWD accumulations with airborne multispectral imagery. Field surveys were conducted on the Snake River, Soda Butte Creek, and Cache Creek in the Greater Yellowstone Ecosystem, USA. The amount of woody debris per kilometer is highest in 2nd order streams, widely variable in 3rd and 4th order streams, and relatively low in the 6th order system. Floods led to increases in woody debris in 2nd order streams. Floods redistributed the wood in 3rd and 4th order streams, removing it from the channel and stranding it on bars, but appeared to generate little change in the total amount of wood throughout the channel system. The movement of woody debris suggests a system that is the reverse of most sediment transport systems in mountains. In 1st and 2nd order tributaries, the wood is too large to be moved and the system is transport-limited, with floods introducing new material through undercutting, but not removing wood through downstream transport. In the intermediate 3rd and 4th order channels, the system displays characteristics of dynamic equilibrium, where the channel is able remove the debris at approximately the same rate that it is introduced. The spatial distribution and quantity of wood in 3rd and 4th order reaches varies widely, however, as wood is alternatively stranded on gravel bars or moved downstream during periods of bar mobilization. In the 6th order and larger channels, the system becomes supply-limited, where almost all material in the main stream can be transported out of the central channel by normal stream flows and deposition occurs primarily on banks or in eddy pool environments. Attempts to map woody debris with 1-m resolution digital four-band imagery were generally unsuccessful, primarily because the imagery could not distinguish the narrow logs within a pixel from the surrounding sand and gravel background and due to problems in precisely coregistering imagery and field maps.     

Riparian reforestation and channel change: A case study of two small tributaries to Sleepers River, northeastern Vermont, USA

Measurements of two small streams in northeastern Vermont, collected in 1966 and 2004–2005, document considerable change in channel width following a period of passive reforestation. Channel widths of several tributaries to Sleepers River in Danville, VT, USA, were previously measured in 1966 when the area had a diverse patchwork of forested and nonforested riparian vegetation. Nearly 40 years later, we remeasured bed widths and surveyed large woody debris (LWD) in two of these tributaries, along 500 m of upper Pope Brook and along nearly the entire length (3 km) of an unnamed tributary (W12). Following the longitudinal survey, we collected detailed channel and riparian information for nine reaches along the same two streams. Four reaches had reforested since 1966; two reaches remained nonforested. The other three reaches have been forested since at least the 1940s. Results show that reforested reaches were significantly wider than as measured in 1966, and they are more incised than all other forested and nonforested reaches. Visual observations, cross-sectional surveys, and LWD characteristics indicate that reforested reaches continue to change in response to riparian reforestation. The three reaches with the oldest forest were widest for a given drainage area, and the nonforested reaches were substantially narrower. Our observations culminated in a conceptual model that describes a multiphase process of incision, widening, and recovery following riparian reforestation of nonforested areas. Results from this case study may help inform stream restoration efforts by providing insight into potentially unanticipated changes in channel size associated with the replanting of forested riparian buffers adjacent to small streams.     

Geomorphic effects of rural-to-urban land use conversion on three streams in the Central Redbed Plains of Oklahoma

This research evaluates the impact of rural-to-urban land use conversion on channel morphology and riparian vegetation for three streams in the Central Redbed Plains geomorphic province (central Great Plains ecoregion) of Oklahoma. The Deep Fork Creek watershed is largely urbanized; the Skeleton Creek watershed is largely rural; and the Stillwater Creek watershed is experiencing a rapid transition from rural to urban land cover. Each channel was divided into reaches based on tributary junctions, sinuosity, and slope. Field surveys were conducted at transects in a total of 90 reaches, including measurements of channel units, channel cross-section at bankfull stage, and riparian vegetation. Historical aerial photographs were available for only Stillwater Creek watershed, which were used to document land cover in this watershed, especially changes in the extent of urban areas (impervious cover).The three streams have very low gradients (< 0.001), width-to-depth ratios < 10, and cohesive channel banks, but have incised into red Permian shales and sandstone. The riparian vegetation is dominated by cottonwoods, ash, and elm trees that provide a dense root mat on stream banks where the riparian vegetation is intact. Channels increased in width and depth in the downstream direction as is normally expected, but the substrate materials and channel units remained unchanged. Statistical analyses demonstrated that urbanization did not explain spatial patterns of changes in any variables. These three channels in the central Redbed Plains are responding as flumes during peak flows, funneling runoff and the wash-load sediment downstream in major runoff events without any effect on channel dimensions. Therefore, local geological conditions (similar bedrock, cohesive substrates and similar riparian vegetation) are mitigating the effects of urbanization.     

Transport and deposition of large woody debris in streams: a flume experiment

Large woody debris (LWD) is an integral component of forested streams of the Pacific Northwest and elsewhere, yet little is known about how far wood is transported and where it is deposited in streams. In this paper, we report the results of flume experiments that examine interactions among hydraulics, channel geometry, transport distance and deposition of floating wood. These experiments were carried out in a 1.22-m-wide×9.14-m-long gravel bed flume using wooden dowels of various sizes as surrogate logs. Channel planforms were either self-formed or created by hand, and ranged from meanders to alternate bars. Floating pieces tended to orient with long axes parallel to flow in the center of the channel. Pieces were deposited where channel depth was less than buoyant depth, typically at the head of mid-channel bars, in shallow zones where flow expanded, and on the outside of bends. We hypothesize that the distance logs travel may be a function of the channel's debris roughness, a dimensionless index incorporating ratios of piece length and diameter to channel width, depth and sinuosity. Travel distance decreased as the ratio of piece length to both channel width and radius of curvature increased, but the relative importance of these variables changed with channel planform. Large pieces can move further than our debris roughness models predict if greater than 50% of the active channel area is deeper than the buoyant depth of the piece, or if momentum is high enough to carry pieces across shallows. Our debris roughness model allows first-order prediction of the amount of wood transport under various channel geometries.     

Distribution and dynamics of large woody debris and organic matter in a low-energy meandering stream

This field-based study, using a combination of bed sediment sampling and large woody debris (LWD) tracking, explores the storage of organic matter and mobility of LWD in Poplar Creek, a low-energy meandering river that contains a diverse arrangement of large woody debris (LWD). This research addresses the following questions: 1) Does LWD influence the spatial pattern of the storage of benthic organic matter in meander bends? 2) How mobile is the LWD in low-gradient meandering systems? 3) Do characteristics of LWD such as length/bankfull channel width ratio, affect movement? Based on detailed sampling of sediment and LWD monitoring, this study provides a detailed representation of the influence of LWD on the storage of benthic organic matter in meander bends, as well as a detailed documentation of the mobility of LWD in the study reach. This study has demonstrated that: 1) LWD strongly influences patterns of storage of benthic organic matter in low-gradient meander bends; 2) removal of LWD from meander bends in low-gradient systems can substantially reduce the storage of benthic organic matter; and 3) the LWD within the study reach at Poplar Creek is highly mobile. The high mobility of LWD, observed in this study, contrasts with the rates observed in high-gradient systems and suggests that LWD-influenced patterns of storage of organic matter are temporally variable and that storage of benthic organic matter occurs in pulses, interspersed with transport, that are keyed to the mobility of LWD. Although LWD may exert an equally significant control over patterns of storage of organic matter in high- and low-gradient systems, the differences in the mobility rates of LWD suggest a fundamental difference in the dynamics of LWD-mediated organic matter between low- and high-gradient systems.     

Influence of large woody debris on morphological evolution of incised,sand-bed channels

This paper documents the influence of Large Woody Debris (LWD) on the morphological evolution of unstable, degrading, sand-bed rivers in the Yazoo Basin, North Mississippi, USA. The study was performed as part of the Demonstration Erosion Control (DEC) project. Twenty-three river reaches were studied, with the aim of determining whether the presence of LWD was beneficial or detrimental to the recovery of stability in degrading, sand-bed river systems and to provide the geomorphic understanding necessary to underpin enhanced LWD management strategies. The results demonstrate that locations of LWD inputs, volumes of LWD stored in different reaches and number of jams per unit channel length are causally related to the morphological processes occurring during different stages of adjustment in these unstable, incised fluvial systems and may be explained using a Channel Evolution Model (CEM). The net impact of LWD jams on reach-scale sediment budgets was found, in general, to be positive: that is, jams trap more sediment than they mobilise. This suggests that LWD probably accelerates rather than retards recovery of a stable longitudinal profile and channel configuration following incision. Field typing of LWD jams, based on their impacts on the flow pattern, reveals that jam type is a function of the size of large, key elements in the jam in relation to the channel width. A Debris Jam Classification Scheme is proposed on this basis, with the spatial relationship between jam type and drainage basin area expressed using a dimensionless function of the ratio between channel width and average riparian tree height. The scheme features four jam types, Underflow, Dam, Deflector and Flow Parallel/Bar Head, each of which has a different morphological impact on local channel geometry. These jam types may be used to classify LWD jams as an aid in determining appropriate management strategies, according to their location within the drainage basin.     

The geomorphic function and characteristics of large woody debris in low gradient rivers, coastal Maine, USA

The role, function, and importance of large woody debris (LWD) in rivers depend strongly on environmental context and land use history. The coastal watersheds of central and northern Maine, northeastern U.S., are characterized by low gradients, moderate topography, and minimal influence of mass wasting processes, along with a history of intensive commercial timber harvest. In spite of the ecological importance of these rivers, which contain the last wild populations of Atlantic salmon (Salmo salar) in the U.S., we know little about LWD distribution, dynamics, and function in these systems. We conducted a cross-basin analysis in seven coastal Maine watersheds, documenting the size, frequency, volume, position, and orientation of LWD, as well as the association between LWD, pool formation, and sediment storage. In conjunction with these LWD surveys, we conducted extensive riparian vegetation surveys. We observed very low LWD frequencies and volumes across the 60 km of rivers surveyed. Frequency of LWD ≥ 20 cm diameter ranged from 15–50 pieces km^− 1 and wood volumes were commonly < 10–20 m³ km^− 1. Moreover, most of this wood was located in the immediate low-flow channel zone, was oriented parallel to flow, and failed to span the stream channel. As a result, pool formation associated with LWD is generally lacking and < 20% of the wood was associated with sediment storage. Low LWD volumes are consistent with the relatively young riparian stands we observed, with the large majority of trees < 20 cm DBH. These results strongly reflect the legacy of intensive timber harvest and land clearing and suggest that the frequency and distribution of LWD may be considerably less than presettlement and/or future desired conditions.     

Interactions between geomorphology and ecosystem processes in travertine streams: Implications for decommissioning a dam on Fossil Creek, Arizona

Travertine deposits of calcium carbonate can dominate channel geomorphology in streams where travertine deposition creates a distinct morphology characterized by travertine terraces, steep waterfalls, and large pools. Algae and microorganisms can facilitate travertine deposition, but how travertine affects material and energy flow in stream ecosystems is less well understood. Nearly a century of flow diversion for hydropower production has decimated the natural travertine formations in Fossil Creek, Arizona. The dam will be decommissioned in 2005. Returning carbonate-rich spring water to the natural stream channel should promote travertine deposition. How will the recovery of travertine affect the ecology of the creek? To address this question, we compared primary production, decomposition, and the abundance and diversity of invertebrates and fish in travertine and riffle/run reaches of Fossil Creek, Arizona. We found that travertine supports higher primary productivity, faster rates of leaf litter decomposition, and higher species richness of the native invertebrate assemblage. Observations from snorkeling in the stream indicate that fish density is also higher in the travertine reach. We postulate that restoring travertine to Fossil Creek will increase stream productivity, rates of litter processing, and energy flow up the food web. Higher aquatic productivity could fundamentally shift the nature of the stream from a sink to a source of energy for the surrounding terrestrial landscape.     

Stream Temperature Spatial Variability Reflects Geomorphology,Hydrology, and Microclimate: Navarro River Watershed,California

Stream temperatures are critical to coldwater fish and vary with microclimate, geomorphology, and hydrology, including influx of groundwater. Spatial variability of stream temperatures was examined at reach and watershed scales within the 816 km² Navarro River watershed in California. Field monitoring and numerical modeling illustrate that stream temperatures were highest at sites with high solar incidence (low shading and wide streams), long travel times, and low discharge. Microclimate helps explain deviation from the general pattern of streams warming with increasing drainage area. Reach-scale field observations of channel width and groundwater influx explain variation in stream temperatures not captured by watershed-scale models.     

Historical Channel-Bed Elevation Change as a Result of Multiple Disturbances,Soldier Creek,Kansas

Historical information on stream stage/discharge relations from eight U.S. Geological Survey (USGS) streamflow-gaging stations was used to analyze channel-bed elevation change along Soldier Creek, a stream affected by multiple disturbances in northeast Kansas. The analysis provided information on the spatial (location, type, magnitude) and temporal (timing, duration, trend, rate) dimensions of channel change. Channel changes determined for Soldier Creek included extensive changes resulting from channelization and changes of relatively limited extent following a flood. The results were used to document channel changes, partly reconstruct historical channel conditions, infer the causes of channel change, and estimate the occurrence of future channel changes.     

Effects of Woody Debris on Channel Morphology and Sediment Storage in Headwater Streams in the Great Smoky Mountains,Tennessee-North Carolina

Coarse woody debris (CWD) is an important component of headwater streams, however, few studies have investigated the geomorphic effects of CWD in the southern Appalachians. In the Great Smoky Mountains, debris slides supply large volumes of CWD and sediment to low-order streams. This study investigates the effect of CWD on bankfull channel dimensions and in-channel sediment storage along second-order streams. Comparisons are made between streams that have experienced recent debris slides and those that have not. CWD channel obstructions are larger but less frequent along debris-slide-affected streams. Dendrochronological evidence indicates that CWD can remain in channels for over 100 yr. Relatively short residence times of CWD along debris-slide-affected streams suggest that logs are frequently flushed through these streams. CWD causes channel widening along all study streams, but the volume of sediment stored in the channel behind CWD obstructions is up to four times greater than the volume of sediment represented by bank erosion associated with CWD. Two large log jams formed by debris slides at tributary junctions stored approximately 4000 m³ of sediment. Sediment stored by CWD was finer than mean bed particle size, and thus represents a significant sediment source when CWD obstructions are breached.     

Bed load transport in an obstruction-formed pool in a forest, gravelbed stream

This paper examines channel dynamics and bed load transport relations through an obstruction-forced pool in a forest, gravel-bed stream by comparing flow conditions, sediment mobility, and bed morphology among transects at the pool head, centre, and tail. Variable sediment supply from within and outside of the channel led to a complex pattern of scour and fill hysteresis. Despite the large flood magnitude, large portions of the bed did not scour. Scour was observed at three distinct locations: two of these were adjacent to large woody debris (LWD), and the third was along the flow path deflected by a major LWD obstruction. Bed material texture showed little change in size distribution of either surface or subsurface material, suggesting lack of disruption of the pre-flood bed. Fractions larger than the median size of the bed surface material were rarely mobile. Sediment rating relations were similar, although temporal variation within and among stations was relatively high. Relations between bed load size distribution and discharge were complex, showing coarsening with increasing discharge followed by fining as more sand was mobilized at high flow. Lack of local scour in the pool combined with bed load fining and net fill by relatively fine material implied that the dominant sources of mobile sediment were upstream storage sites and local bank collapse. Patterns of flow, channel dynamics, and sediment mobility were strongly affected by a LWD flow obstruction in the pool centre that created turbulent effects, thereby enhancing entrainment and transport in a manner similar to scour at bridge piers.     

GEOMORPHIC RESPONSE TO WILDFIRE IN AN ARID WATERSHED,CROW CANYON,NEVADA

On August 28, 1981, the Crow Canyon drainage basin in central Nevada was burned by a lightning-generated wildfire that destroyed the vegetation cover consisting primarily of juniper trees, sagebrush, and desert grasses. The geomorphic impact of the wildfire was assessed on the basis of aerial photography, measurements of sediment movement on hillslopes using charred tree trunks as erosion indicators, and surveys of the valley floor, axial channel, and alluvial fan. Aerial photographs indicate the valley floor was untrenched prior to the fire. The combination of foliage destruction and heavy runoff in the spring following the wildfire initiated channel downcutting that has now reached as much as 3.9 m in depth. Entrenchment of the valley-fill in the lower 2.2 km of the drainage network produced as much as 48, 142 m³ of sediment. Much of the channel incision occurred during 1982 and 1983, years characterized by above-normal precipitation. Approximately 17,608 m³ of sediment were deposited on a preexisting alluvial fan at the mouth of the basin. Following initial channel entrenchment and deposition on the fan, a spatially out-of-phase episode of channel cutting was initiated on the fan apex, a process that is redistributing sediment down-fan. Thus, one geomorphic disturbance has produced two discrete depositional events on the fan. Moreover, the geomorphic instability was still evident over a decade after the wildfire. [Key words: wildfire, degradation, channel entrenchment, soil erosion, complex-response.]     

Late Quaternary Alluviation Along Intermittent Streams in Kakadu National Park,Northern Territory

Sedimentation along small, intermittent streams on Kapalga Research Station in Kakadu National Park may have responded to increased base levels following post-glacial flooding of the valleys of the Alligator Rivers. Alternatively, regional climate changes may have controlled sedimentation. Using thermoluminescence dating, we determined that sediments from two streams at Kapalga date from 21.5 ± 4.0 ka. On a third stream, sediments dated from 7.6 ± 1.1 ka, with younger sediments occurring downstream. We interpreted the pre-Holocene dates and the lack of evidence of upstream progradation to indicate that climate variation was more important to sedimentation than base levels. Predicted increases in rainfall variability and in the frequency of high-intensity rainfall under enhanced greenhouse conditions may cause renewed sediment mobilisation. At the outflow of one stream on to the South Alligator flood plain, we found 15 m of sandy alluvia underlying 3-5 m of estuarine muds deposited as a result of sea-level rise. These sandy alluvia dated from about 77 ka at 4 m to more than 300 ka at 19 m depth. These ages are consistent with those recorded on the Magela Creek system, 50 km to the east.     

Geomorphic Assessment of the Effects of Flow Diversion on Anadromous Fish Spawning Habitat: Newhalem Creek,Washington*

A simple model for estimating streamflow competence is used to assess the impact of water diversion on the retention of spawning gravels in Newhalem Creek, a small watershed in the Cascade Mountains. Surveyed stream cross sections and discharge estimates are employed to characterize the routine tractive force conditions of the streambed. A conservative estimation procedure results in tractive force values of 120–390 Nm^?2for one-year, five-year, and 10-year recurrence interval events. Theoretical and empirical estimates of critical tractive force for particle sizes suggest a routine competence of 100–450 mm diameter for these flows. Substantial accumulations of bed material in the range of 1–100 mm diameter is desirable for anadromous fish spawning redds. As a result, Newhalem Creek is naturally poor habitat for spawning. Steep channel gradients and a “flashy” hydrograph conspire to routinely flush the main channel of spawning-size substrate. In this instance, the modest water diversion is immaterial to the maintenance of spawning habitat.     

Stream response to repeated coseismic folding, Tiptonville dome, New Madrid seismic zone

Fluvial response to tectonic deformation is dependent on the amount and style of surface deformation and the relative size of the stream. Active folding in the New Madrid seismic zone (NMSZ) forms the Tiptonville dome, a 15-km long and 5-km wide surface fold with up to 11 m of late Holocene structural relief. The fold is crossed by streams of varying size, from the Mississippi River to small flood-plain streams. Fluvial response of these streams to repeated coseismic folding has only been preserved for the past 2.3 ka, since the Tiptonville meander of the Mississippi River migrated across the area forming the present flood plain. This surface comprises a sandy point-bar deposit locally overlain by clayey overbank and silty sand crevasse-splay deposits, an abandoned chute channel infilled with laminated sandy silt and silty clay, and an abandoned neck cutoff filled with a sandy cutoff bar and silty clay oxbow lake deposits.Dating various stream responses to coseismic folding has more tightly constrained the timing of earthquake events in the central NMSZ and provides a means of partitioning the deformation amount into individual seismic events. Three earthquakes have been dated in the Reelfoot Lake area, ca. A.D. 900, 1470, and 1812. The latter two earthquakes had large local coseismic deformation. Both of these events were responsible for numerous stream responses such as shifting depocenters, modification of Mississippi River channel geometry, and derangement of small streams. Overbank sedimentation ceased on the dome as it was uplifted above the normal flood stage, and sedimentation of crevasse-splay deposits from the Mississippi River, colluvium from the scarp, and lacustrine sediment accumulated in the adjacent Reelfoot basin. The much larger Mississippi River channel responded to uplift by increasing its sinuosity across the uplift relative to both upstream and downstream, increasing its width/depth ratio across and downstream of the uplift, and decreasing the width/depth ratio upstream of the uplift. Despite the size of the Mississippi River, it has not yet attained equilibrium since the latest uplift 190 years ago. Small channels that could not downcut through the uplift were filled, locally reversed flow direction, or formed a lake where they were dammed.Uplift and stream response to folding along the Tiptonville dome is less dramatic between 2.3 and 0.53 ka. During this interval, abandoned channel fill and overbank deposition across the dome suggests that it was not a high-relief feature. One earthquake event occurred during this interval (ca. A.D. 900), but coseismic stream response was probably limited to a slight aggradation of a small flood-plain stream.     

Biogeomorphic Relationships and Riparian Vegetation Changes Along Altered Ephemeral Stream Channels: Florence to Marana,Arizona

This study examines riparian vegetation cover changes along ephemeral channels due to the emplacement of the Central Arizona Project (CAP) canal. Two research questions examined are the following: (1) How has riparian vegetation changed over the course of twenty-eight years due to altered flow conditions? (2) How has channel morphology affected changes in vegetation cover? Five Landsat TM images acquired in 1982, 1989, 1996, 2003, and 2010 were classified. The average change of vegetation cover per 0.5-km section over the twenty-eight-year period is approximately 100,436 m² over 25.5-km length of the canal on the upstream section. In addition, the total amount of vegetation cover increase in the twenty-eight years over the 25.5-km length of the canal is approximately 5,122,239 m². Larger streams experienced a greater increase in vegetation cover upslope than smaller streams. In addition, streams of similar width dimensions that were completely closed off resulted in greater vegetation cover than streams that were semiconnected. A significant relationship between changes in vegetation green-up and channel widths was examined. Results from this study suggest that there is a quasi-linear relationship between channel widths and increases in vegetation cover for altered and impounded channels due to the presence of the CAP canal.     

Human impacts to mountain streams

Mountain streams are here defined as channel networks within mountainous regions of the world. This definition encompasses tremendous diversity of physical and biological conditions, as well as history of land use. Human effects on mountain streams may result from activities undertaken within the stream channel that directly alter channel geometry, the dynamics of water and sediment movement, contaminants in the stream, or aquatic and riparian communities. Examples include channelization, construction of grade-control structures or check dams, removal of beavers, and placer mining. Human effects can also result from activities within the watershed that indirectly affect streams by altering the movement of water, sediment, and contaminants into the channel. Deforestation, cropping, grazing, land drainage, and urbanization are among the land uses that indirectly alter stream processes. An overview of the relative intensity of human impacts to mountain streams is provided by a table summarizing human effects on each of the major mountainous regions with respect to five categories: flow regulation, biotic integrity, water pollution, channel alteration, and land use. This table indicates that very few mountains have streams not at least moderately affected by land use. The least affected mountainous regions are those at very high or very low latitudes, although our scientific ignorance of conditions in low-latitude mountains in particular means that streams in these mountains might be more altered than is widely recognized. Four case studies from northern Sweden (arctic region), Colorado Front Range (semiarid temperate region), Swiss Alps (humid temperate region), and Papua New Guinea (humid tropics) are also used to explore in detail the history and effects on rivers of human activities in mountainous regions. The overview and case studies indicate that mountain streams must be managed with particular attention to upstream/downstream connections, hillslope/channel connections, process domains, physical and ecological roles of disturbance, and stream resilience.     

Downstream variation in bankfull width of wadeable streams across the conterminous United States

Bankfull channel width is a fundamental measure of stream size and a key parameter of interest for many applications in hydrology, fluvial geomorphology, and stream ecology. We developed downstream hydraulic geometry relationships for bankfull channel width w as a function of drainage area A, w = α A^β, (DHG_wA) for nine aggregate ecoregions comprising the conterminous United States using 1588 sites from the U.S. Environmental Protection Agency's National Wadeable Streams Assessment (WSA), including 1152 sites from a randomized probability survey sample. Sampled stream reaches ranged from 1 to 75 m in bankfull width and 1 to 10,000 km² in drainage area. The DHG_wA exponent β, which expresses the rate at which bankfull stream width scales with drainage area, fell into three distinct clusters ranging from 0.22 to 0.38. Width increases more rapidly with basin area in the humid Eastern Highlands (encompassing the Northern and Southern Appalachians and the Ozark Mountains) and the Upper Midwest (Great Lakes region) than for the West (both mountainous and xeric areas), the southeastern Coastal Plain, and the Northern Plains (the Dakotas and Montana). Stream width increases least rapidly with basin area in the Temperate Plains (cornbelt) and Southern Plains (Great Prairies) in the heartland. The coefficient of determination (r²) was least in the noncoastal plains (0.36–0.41) and greatest in the Appalachians and Upper Midwest (0.68–0.77). DHG_wA equations differed between streams with dominantly fine bed material (silt/sand) and those with dominantly coarse bed material (gravel/cobble/boulder) in six of the nine analysis regions. Where DHG_wA equations varied by sediment size, fine-bedded streams were consistently narrower than coarse-bedded streams. Within the Western Mountains ecoregion, where there were sufficient sites to develop DHG_wA relationships at a finer spatial scale, α and β ranged from 1.23 to 3.79 and 0.23 to 0.40, respectively, with r² > 0.50 for 10 of 13 subregions (range: 0.36 to 0.92). Enhanced DHG equations incorporating additional data for three landscape variables that can be derived from GIS—mean annual precipitation, elevation, and mean reach slope—significantly improved equation fit and predictive value in several regions, most notably the Western Mountains and the Temperate Plains. Channel width was also related to human disturbance. We examined the influence of human disturbance on channel width using several indices of local and basinwide disturbance. Contrary to our expectations, the data suggest that the dominant response of channel width to human disturbance in the United States is a reduction in bankfull width in streams with greater disturbance, particularly in the Western Mountains (where population density, road density, agricultural land use, and local riparian disturbance were all negatively related to channel width) and in the Appalachians and New England (where urban and agricultural land cover and riparian disturbance were all negatively associated with channel width).     

Effects of fire, grazing, and the presence of shrubs on Chihuahuan desert grasslands

Responses of herbaceous and suffrutescent species to fire, grazing, and presence of Prosopis glandulosa were examined in a Chihuahuan desert grassland in south-central New Mexico. Treatments were assigned randomly to eight 12×8 m plots within each of two blocks. Following fires in June 1995, unfenced plots were exposed to livestock grazing over 4 years. Plots were established that either included or excluded P. glandulosa. Perennial grass cover, primarilyBouteloua eriopoda , decreased by 13% in burned plots but increased 5% in unburned areas. Conversely, perennial forb cover was 4% greater after fire. Perennial grass frequency decreased 30% more and perennial forb frequency increased 10% more following burning. Further, increases in evenness after fire resulted in a 225% increase in species diversity. Grazing also resulted in a decrease in perennial grass cover while frequency decreased 22% more in grazed than ungrazed plots. Only frequency and not cover of perennial forbs and annual grasses increased more following grazing. Presence of P. glandulosa had no differential effect on responses of non-shrub species. Fires were conducted during near drought conditions while grazing occurred during years of precipitation equivalent to the long-term average. Precipitation immediately following fire may be critical for recovery of B. eriopoda -dominated desert grasslands; relationships between fire and post-fire precipitation patterns require future investigation.