Quaternary landscape evolution and erosion rates for an intramontane Neogene basin (Guadix–Baza basin, SE Spain)

The landscape evolution in Neogene intramontane basins is a result of the interaction of climatic, lithologic, and tectonic factors. When sedimentation ceases and a basin enters an erosional stage, estimating erosion rates across the entire basin can offer a good view of landscape evolution. In this work, the erosion rates in the Guadix–Baza basin have been calculated based on a volumetric estimate of sediment loss by river erosion since the Late Pleistocene. To do so, the distribution of a glacis surface at ca. 43 kyr, characterised by a calcrete layer that caps the basin infilling, has been reconstructed. To support this age, new radiometric data of the glacis are presented. The volume of sediment loss by water erosion has been calculated for the entire basin by comparing the reconstructed geomorphic surface and the present-day topography. The resulting erosion rates vary between 4.28 and 6.57 m³ ha^− 1 yr^− 1, and are the consequence of the interaction of climatic, lithologic, topographic, and tectonic factors. Individual erosion rates for the Guadix and Baza sub-basins (11.80 m³ ha^− 1 yr^− 1 and 1.77 m³ ha^− 1 yr^− 1 respectively) suggest different stages of drainage pattern evolution in the two sub-basins. We attribute the lower values obtained in the Baza sub-basin to the down-throw of this sub-basin caused by very recent activity along the Baza fault.     

Simulation of soil erosion and deposition in a changing land use: A modelling approach to implement the support practice factor

Using the USPED (Unit Stream Power Erosion Deposition) model, three land use scenarios were analysed for an Italian small catchment (15 km²) of high landscape value. The upper Orme stream catchment, located in the Chianti area, 30 km south of Florence, has a long historical agriculture record. Information on land use and soil conservation practices date back to 1821, hence offering an opportunity to model impacts of land use change on erosion and deposition. For this study, a procedure that takes into account soil conservation practices and potential sediment storage is proposed. The approach was to calculate and model the flow accumulation considering rural and logging roads, location of urban areas, drainage ditches, streams, gullies and permanent sediment sinks. This calculation attempts to assess the spatial variability, especially the impact of support practices (P factor). Weather data from 1980–2003 were taken into account to calculate the R factor. However, to consider the intense pluviometric conditions in terms of the erosivity factor, the 0.75th quantile was used, while the lowest erosivity was modelled using the 0.25th quantile. Results of the USPED model simulation show that in 1821 the mean annual net erosion for the watershed was 2.8 Mg ha^− 1 y^− 1; in 1954 it was 4.2 Mg ha^− 1 y^− 1; and in 2004 it was 5.3 Mg ha^− 1 y^− 1. Conservation practices can reduce erosion processes by ≥ 20 Mg ha^− 1 y^− 1 when the 1821 practices are introduced in the present management. On the other hand, if the support practices are not considered in the model, soil erosion risk is overestimated. Field observation for the present-day simulation confirmed that erosion and associated sediment deposition predicted by the model depend, as expected, on geomorphology and land use. The model shows limitations that are mainly due to the input data. A high resolution DEM is essential for the delineation of reliable topographic potential to predict erosion and deposition especially in vineyards.     

Environmental factors controlling spatial variation in sediment yield in a central Andean mountain area

A large spatial variability in sediment yield was observed from small streams in the Ecuadorian Andes. The objective of this study was to analyze the environmental factors controlling these variations in sediment yield in the Paute basin, Ecuador. Sediment yield data were calculated based on sediment volumes accumulated behind checkdams for 37 small catchments. Mean annual specific sediment yield (SSY) shows a large spatial variability and ranges between 26 and 15,100 Mg km^− 2 year^− 1. Mean vegetation cover (C, fraction) in the catchment, i.e. the plant cover at or near the surface, exerts a first order control on sediment yield. The fractional vegetation cover alone explains 57% of the observed variance in ln(SSY). The negative exponential relation (SSY = a × e^{−b C}) which was found between vegetation cover and sediment yield at the catchment scale (10³–10⁹ m²), is very similar to the equations derived from splash, interrill and rill erosion experiments at the plot scale (1–10³ m²). This affirms the general character of an exponential decrease of sediment yield with increasing vegetation cover at a wide range of spatial scales, provided the distribution of cover can be considered to be essentially random. Lithology also significantly affects the sediment yield, and explains an additional 23% of the observed variance in ln(SSY). Based on these two catchment parameters, a multiple regression model was built. This empirical regression model already explains more than 75% of the total variance in the mean annual sediment yield. These results highlight the large potential of revegetation programs for controlling sediment yield. They show that a slight increase in the overall fractional vegetation cover of degraded land is likely to have a large effect on sediment production and delivery. Moreover, they point to the importance of detailed surface vegetation data for predicting and modeling sediment production rates.     

Rates of sheet and rill erosion in Germany — A meta-analysis

Knowledge of erosion rates under real conditions is of great concern regarding sustainability of landuse and off-site effects on water bodies and settlements. Experimentally derived rates of sheet and rill erosion are often biased by experimental settings, which deviate considerably from typical landuse, by short measuring periods and by small spatial extensions, which do not account for the pronounced spatio-temporal variability of erosion events. We compiled data from 27 studies covering 1076 plot years to account for this variability. Modelling was used to correct for deficiencies in the experimental settings, which overrepresented arable land and used steeper and shorter slopes as well as higher erosivity than typically found in reality. For example, the average slope gradient was 5.9° for all arable plot experiments while it is only 2.6° on total arable land in Germany. The expected soil loss by sheet and rill erosion in Germany after taking real slopes, landuse and erosivity into account averaged 2.7 t ha^− 1 yr^− 1. Annual crops contributed the largest proportion (90%) but hops despite its negligible contribution to landuse (0.06%) still contribute 1.0% due to its extraordinary rapid erosion, which was even faster than the measured bare fallow soil loss standardized to otherwise identical conditions. Bare fallow soil loss, which is often used as baseline, was 80 t ha^− 1 yr^− 1 when standardized to 5.1° slope gradient, 200 m flow path length, and average German erosivity.     

Erosion rates and sediment budgets in vineyards at 1-m resolution based on stock unearthing (Burgundy, France)

A new and simple method is developed to efficiently quantify erosion and deposition rates based on stock unearthing measurements. This is applicable to spatial scales ranging from plot to hillslopes, and to time scales ranging from single hydrologic events to centennial scales. The method is applied to a plot area on vineyard hillslopes in Burgundy (Monthélie, France), with measurement of 4328 vine plants. A sediment budget established at the plot scale shows a mean soil lowering of 3.44 ± 1 cm over 20 years, involving a minimal erosion rate of 1.7 ± 0.5 mm yr^− 1. Locally, erosion rates can reach up to 8.2 ± 0.5 mm yr^− 1.This approach allows the sediment redistribution to be mapped and analyzed at 1-m resolution. It provides novel insights into the characterization of erosion patterns on pluri-decennial scales and into the analysis of spatial distribution of erosion processes on cultivated hillslopes.     

Development of gullies and sediment production in the black soil region of northeastern China

Gully erosion is an important environmental hazard in the black soil region of northeastern China. It is a primary sediment source in the region which needs appropriate soil conservation practices. Gully incision in rolling hills typical of this region was monitored using real-time kinematic GPS to assess the rates of gully development and the resultant sediment production. From 2002 to 2005, gully heads in the study area retreated between 15.4 and 33.5 m, giving an average retreat rate of 8.4 m yr^− 1. Field measurements showed that total sediment production due to gully erosion during the three years ranged between 257 and 1854 m³ yr^− 1, which is equivalent to 326 to 2355 t yr^− 1, with gully-head retreat accounting for 0 to 21.7% (4.4% in average). The sediment delivery ratio was especially high during the summer rainy season (56% in average). Sediment production by ephemeral gullies and permanent gullies was 1.5 times greater than that from surface erosion. Gully heads retreated faster in the spring freeze–thaw period than in the summer. The stage of gully development could be identified based on short-term changes in the gully erosion rate.     

Deep-seated failure propagation in a fractured rock slope over 10,000 years: The La Clapière slope, the south-eastern French Alps

The “La Clapière” area (Tinée valley, Alpes Maritimes, France) is a typical large, complex, unstable rock slope affected by Deep Seated Gravitational Slope Deformations (DGSD) with tension cracks, scarps, and a 60 × 10⁶ m³ rock slide at the slope foot that is currently active. The slope surface displacements since 10 ka were estimated from ¹⁰Be ages of slope gravitational features and from morpho-structural analyses. It appears that tensile cracks with a strike perpendicular to the main orientation of the slope were first triggered by the gravitational reactivation of pre-existing tectonic faults in the slope. A progressive shearing of the cracks then occurred until the failure of a large rock mass at the foot of the slope. By comparing apertures, variations and changes in direction between cracks of different ages, three phases of slope surface displacement were identified: 1) an initial slow slope deformation, spreading from the foot to the top, characterized by an average displacement rate of 4 mm yr^− 1, from 10–5.6 ka BP; 2) an increase in the average displacement rate from 13 to 30 mm yr^− 1 from the foot to the middle of the slope, until 3.6 ka BP; and 3) development of a large failure at the foot of the slope with fast displacement rates exceeding 80 mm yr^− 1 for the last 50 years. The main finding of this study is that such a large fractured slope destabilization had a very slow displacement rate for thousands of years but was followed by a recent acceleration. The results obtained agree with several previous studies, indicating that in-situ monitoring of creep of a fractured rock slope may be useful for predicting the time and place of a rapid failure.     

Processes and rates of rock fragment displacement on cliffs and scree slopes in an amba landscape, Ethiopia

Distinct rock fragment displacements occur on the ambas, or structurally determined stepped mountains of the Northern Ethiopian Highlands. This paper describes the rock fragment detachment from cliffs by rockfall, quantifies its annual rate, and identifies factors controlling rock fragment movement on the scree slopes. It further presents a conceptual model explaining rock fragment cover at the soil surface in these landscapes. In the May Zegzeg catchment (Dogu'a Tembien district, Tigray), rockfall from cliffs and rock fragment movement on debris slopes by runoff and livestock trampling were monitored over a 4-year period (1998–2001). Rockfall and rock fragment transport mainly induced by livestock trampling appear to be important geomorphic processes. Along a 1500-m long section of the Amba Aradam sandstone cliff, at least 80 t of rocks are detached yearly and fall over a mean vertical distance of 24 m resulting in a mean annual cliff retreat rate of 0.37 mm y^− 1. Yearly unit rock fragment transport rates on scree slopes ranged between 23.1 and 37.9 kg m^− 1 y^− 1. This process is virtually stopped when exclosures are established. Corresponding mean rock fragment transport coefficients K are 32–69 kg m^− 1 y^− 1 on rangeland but only 3.9 kg m^− 1 y^− 1 in densely vegetated exclosures. A conceptual model indicates that besides rockfall from cliffs and argillipedoturbation, all factors and processes of rock fragment redistribution in the study area are of anthropogenic origin.     

Suspended sediment transport in a highly erodible catchment: The River Isábena (Southern Pyrenees)

Understanding and quantifying sediment load is important in catchments draining highly erodible materials that eventually contribute to siltation of downstream reservoirs. Within this context, the suspended sediment transport and its temporal dynamics have been studied in the River Isábena (445 km², south-central Pyrenees, Ebro basin) by means of direct sampling and turbidity recording during a 3-year dry period. The average flood-suspended sediment concentration was 8 g l^− 1, with maximum instantaneous values above 350 g l^− 1. The high scatter between discharge and suspended sediment concentrations (up to five orders of magnitude) has not permitted the use of rating curve methods to estimate the total load. Interpolation techniques yielded a mean annual sediment load of 184,253 t y^− 1 for the study period, with a specific yield of 414 t km^− 2 y^− 1. This value resembles those reported for small torrents in nearby mountainous environments and is the result of the high connectivity between the badland source areas and stream courses, a fact that maximises sediment conveyance through the catchment. Floods dominated the sediment transport and yield. However, sediment transport was more constant through time than that observed in Mediterranean counterparts; this can be attributed to the role of base flows that entrain fine sediment temporarily stored in the channel and force the river to carry high sediment concentrations (i.e., generally in the order of 0.5 g l^− 1), even under minimum flow conditions.     

Occurrence of soil erosion after repeated experimental fires in a Mediterranean environment

In the Mediterranean area, forest fires have become a first-order environmental problem. Increased fire frequency progressively reduces ecosystem recovery periods. The fire season, usually followed by torrential rains in autumn, intensifies erosion processes and increases desertification risk. In this work, the effect of repeated experimental fires on soil response to water erosion is studied in the Permanent Field Station of La Concordia, Valencia, Spain. In nine 80 m² plots (20 m long × 4 m wide), all runoff and sediment produced were measured after each rainfall event. In 1995, two fire treatments with the addition of different biomass amounts were applied. Three plots were burned with high fire intensity, three with moderate intensity, and three were unburned to be used as control. In 2003, the plots with the fire treatments were burned again with low fire intensities. During the 8-year interval between fires, plots remained undisturbed, allowing regeneration of the vegetation–soil system. Results obtained during the first 5 months after both fire experiments show the high vulnerability of the soil to erosion after a repeated fire. For the burned plots, runoff rates increased three times more than those of 1995, and soil losses increased almost twice. The highest sediment yield (514 g m^− 2) was measured in 2003, in the plots of the moderate fire intensity treatment, which yielded only 231 g m^− 2 of sediment during the corresponding period in 1995. Runoff yield from the control plots did not show significant temporal changes, while soil losses decreased from 5 g m^− 2 in the first post-fire period to 0.7 g m^− 2 in the second one.     

Kinematics of a landslide derived from archival photogrammetry and GPS data

The evolution of the Patigno landslide, a deep-seated gravitational slope deformation in the Northern Apennine range (Italy), was investigated using archival photogrammetry, and GPS observations from a permanent station located inside the landslide. Historical aerial photographs of the area taken in 1975 (scale 1:13,000), 1987 (1:13,000) and 2004 (1:30,000) were co-registered into the same reference frame using an unconventional method based on the detection of homologous points in multi-temporal models. Three DTMs were produced using a Digital Photogrammetric Workstation and compared. The displacement vectors of 293 points in the landslide were determined. The average movement velocity of the GPS station since 2004 (about 3.5 cm yr^− 1) agrees with the mean displacement rate obtained from photogrammetry. Furthermore, the estimated velocity varies seasonally between 3 and 6 cm yr^− 1. This variability correlated with rainfall suggests that the creep of the landslide is influenced by hydrological factors.     

Changes in ecosystem carbon stocks following grassland afforestation of semiarid sandy soil in the southeastern Keerqin Sandy Lands, China

The ecological consequences of grassland afforestation in arid/semiarid sandy regions are not well known with respect to tree species and stand age. The present study quantifies the changes in above- and belowground carbon (C) stocks following afforestation in the southeastern Keerqin Sandy Lands with species of Mongolian pine and poplar. We studied 15-, 24-, and 30-year-old Mongolian pine plantations, 7-, 11-, and 15-year-old poplar plantations, and adjacent grasslands. The results show that total ecosystem C stocks increased following grassland afforestation. Aboveground C stocks increased at a rate of 2.75 Mg C ha⁻¹ yr⁻¹ in the poplar plantations, and 1.06 Mg C ha⁻¹ yr⁻¹ in the Mongolian pine plantations. Mineral soil C stocks decreased during the early stage of forest establishment, but recovered with increasing stand age. Root C stock increased significantly in the Mongolian pine plantations, but the poplar plantations showed no such increase relative to the grassland. Our results indicate that afforestation of the grassland in the southeastern Keerqin Sandy Lands would sequester more C than would continuous grassland. Tree species selection and stand developmental age should be considered in planning future afforestation projects.     

Deriving hillslope sediment budgets in wildfire-affected forests using fallout radionuclide tracers

Information on post-fire sediment and nutrient redistribution is required to underpin post-fire catchment management decisions. Fallout radionuclide budgets (²¹⁰Pb_xs, ¹³⁷Cs and ⁷Be) were derived to quantify soil redistribution and sediment yield in forested terrain following a moderately severe wildfire in a small (89 ha) water supply catchment in SE Australia. Application of these techniques in burnt terrain requires careful consideration of the partitioning of radionuclides between organic and mineral soil components. Beryllium-7 and ²¹⁰Pb_xs were shown to be closely associated with ash, litter and soil organic matter whereas ¹³⁷Cs was more closely associated with subsurface coarse mineral soil. Comparison of the three tracer budgets indicated that the dominant sediment source areas were ridgetops and steep valley sideslopes, from which burnt surface material was conveyed to the stream network via pre-existing gullies. Erosion was predominantly driven by sheetwash, enhanced by soil water repellency, and modified by bioturbation which both supplies subsurface sediment and provides sinks for erosive overland flow. Footslope and riparian zones were not important sediment source areas. The estimated event-based (wildfire and subsequent rainfall) sediment yield is 58 ± 25 t km^− 2, based on fallout ⁷Be measurements. The upper estimate of total particulate phosphorus yield (0.70 kg ha^− 1) is more than 10 times that at equivalent unburnt sites. This illustrates that, soon after fire, burnt eucalypt forest can produce nutrient loads similar to those of agricultural catchments. The tracer budgets indicate that wildfire is an important control on sediment and phosphorus inputs to the stream network over the decadal timeframe and the pulsed nature of this release is an important concern for water quality management.     

Groundwater control on the suspended sediment load in the Na Borges River, Mallorca, Spain

Groundwater dominance has important effects on the hydrological and geomorphological characteristics of river systems. Low suspended sediment concentrations and high water clarity are expected because significant inputs of sediment-free spring water dilute the suspended sediment generated by storms. However, in many Mediterranean rivers, groundwater dominance is characterised by seasonal alternations of influent and effluent discharge involving significant variability on the sediment transport regimes. Such areas are often subject to soil and water conservation practices over the centuries that have reduced the sediment contribution from agricultural fields and favour subsurface flow to rivers. Moreover, urbanisation during the twentieth century has changed the catchment hydrology and altered basic river processes due to its ‘flashy’ regime. In this context, we monitored suspended sediment fluxes during a two-year period in the Na Borges River, a lowland agricultural catchment (319 km²) on the island of Mallorca (Balearic Islands). The suspended sediment concentration (SSC) was lower when the base flow index (i.e., relative proportion of baseflow compared to stormflow, BFI) was higher. Therefore, strong seasonal contrasts explain the high SSC coefficient of variation, which is clearly related to dilution effects associated with different groundwater and surface water seasonal interactions. A lack of correlation in the Q-SSC rating curves shows that factors other than discharge control sediment transport. As a result, at the event scale, multiple regressions illustrate that groundwater and surface water interactions are involved in the sedimentary response of flood events. In the winter, the stability of baseflow driven by groundwater contributions and agricultural and urban spills causes hydraulic variables (i.e., maximum discharge) to exert the most important control on events, whereas in the summer, it is necessary to accumulate important volumes of rainfall, creating a minimum of wet conditions in the catchment to activate hydrological pathways and deliver sediment to the drainage network. The BFI is also related to sediment delivery processes, as the loads are higher with lower BFI, corroborating the fact that most sediment movement is caused by stormflow and its related factors. Overall, suspended sediment yields were very low (i.e., < 1 t km^− 2 yr^− 1) at all measuring sites. Such values are the consequence of the limited sediment delivery attributable to soil conservation practices, low surface runoff coefficients and specific geomorphic features of groundwater-dominated rivers, such as low drainage density, low gradient, steep valley walls and flat valley floors.     

Dust emission by off-road driving: Experiments on 17 arid soil types, Nevada, USA

Field experiments were conducted in Nellis Dunes Recreational Area (Clark County, Nevada, USA) to investigate emission of dust produced by off-road driving. Experiments were carried out with three types of vehicles: 4-wheelers (quads), dirt bikes (motorcycles) and dune buggies, on 17 soil types characteristic for a desert environment. Tests were done at various driving speeds, and emissions were measured for a large number of grain size fractions. This paper reports the results for two size fractions of emissions: PM10 (particles < 10 μm) and PM60 (particles < 60 μm). The latter was considered in this study to be sufficiently representative of the total suspendable fraction (TSP). Off-road driving was found to be a significant source of dust. However, the amounts varied greatly with the type of soil and the characteristics of the top layer. Models predicting emission of dust by off-road driving should thus consider a number of soil parameters and not just one key parameter. Vehicle type and driving speed are additional parameters that affect emission. In general, 4-wheelers produce more dust than dune buggies, and dune buggies, more than dirt bikes. Higher speeds also result in higher emissions. Dust emitted by off-road driving is less coarse than the parent sediment on the road surface. Off-road driving thus results in a progressive coarsening of the top layer. Exceptions to this are silty surfaces with no, or almost no, vegetation. For such surfaces no substantial differences were observed between the grain size distribution of road dust and emitted dust. Typical emission values for off-road driving on dry desert soils are: for sandy areas, 30–40 g km^− 1 (PM10) and 150–250 g km^− 1 (TSP); for silty areas, 100–200 g km^− 1 (PM10) and 600–2000 g km^− 1 (TSP); for drainages, 30–40 g km^− 1 (PM10) and 100–400 g km^− 1 (TSP); and for mixed terrain, 60–100 g km^− 1 (PM10) and 300–800 g km^− 1 (TSP). These values are for the types of vehicles tested in this study and do not refer to cars or trucks, which produce significantly more dust.     

Initiation conditions for debris flows generated by runoff at Chalk Cliffs, central Colorado

We have monitored initiation conditions for six debris flows between May 2004 and July 2006 in a 0.3 km² drainage basin at Chalk Cliffs; a band of hydrothermally-altered quartz monzonite in central Colorado. Debris flows were initiated by water runoff from colluvium and bedrock that entrained sediment from rills and channels with slopes ranging from about 14° to 45°. The availability of channel material is essentially unlimited because of thick channel fill and refilling following debris flows by rock fall and dry ravel processes. Rainfall exceeding I = 6.61(D)^− 0.77, where I is rainfall intensity (mm/h), and D is duration (h), was required for the initiation of debris flows in the drainage basin. The approximate minimum runoff discharge from the surface of bedrock required to initiate debris flows in the channels was 0.15 m³/s. Colluvium in the basin was unsaturated immediately prior to (antecedent) and during debris flows. Antecedent, volumetric moisture levels in colluvium at depths of 1 cm and 29 cm ranged from 4–9%, and 4–7%, respectively. During debris flows, peak moisture levels in colluvium at depths of 1 cm and 29 cm ranged from 10–20%, and 4–12%, respectively. Channel sediment at a depth of 45 cm was unsaturated before and during debris flows; antecedent moisture ranged from 20–22%, and peak moisture ranged from 24–38%. Although we have no measurements from shallow rill or channel sediment, we infer that it was unsaturated before debris flows, and saturated by surface-water runoff during debris flows.Our results allow us to make the following general statements with regard to debris flows generated by runoff in semi-arid to arid mountainous regions: 1) high antecedent moisture levels in hillslope and channel sediment are not required for the initiation of debris flows by runoff, 2) locations of entrainment of sediment by successive runoff events can vary within a basin as a function of variations in the thickness of existing channel fill and the rate of replenishment of channel fill by rock fall and dry ravel processes following debris flows, and 3) rainfall and simulated surface-water discharge thresholds can be useful in understanding and predicting debris flows generated by runoff and sediment entrainment.     

Suspended sediment and Cs fluxes during the exceptional December 2003 flood in the Rhone River, southeast France

At the beginning of December 2003, one of the biggest floods for at least 150 yr was recorded on the Rhone River. In the lower part of the river, the peak flood reached 11,000 m³ s⁻¹. The geomorphological and radioecological consequences of such an event were investigated downstream all the nuclear installations by using measured and calculated fluxes and the total export of suspended sediment and associated ¹³⁷Cs. Results pointed out the major role played by large floods in the annual suspended sediment load, as 3.70 × 10⁶ tons of silts, 0.85 × 10⁶ tons of sands, and 0.84 × 10⁶ tons of clays were transferred towards the coastal environment. Nevertheless, these solid loads were found to be lower than those expected as regards the liquid discharge reached during this event and suggested that previous floods that occurred on the river and on its main tributaries during the last decade have probably led to the removal of available sediment from the channels and their banks. Besides, the ¹³⁷Cs activity measured within the suspended load was estimated at 14.9 ± 0.4 Bq kg⁻¹, which is a level characteristic of the suspended sediments from the Rhone catchment area and demonstrated that nuclear installations located along the Rhone valley did not significantly contribute to any increase in ¹³⁷Cs activity in the water during the flood. The total ¹³⁷Cs particulate export amounted to 77 ± 17 GBq and was mainly associated with the silt fraction that contributes to around 70% of the total ¹³⁷Cs export.     

Beach–dune interactions on the dry–temperate Danube delta coast

Beach–dune seasonal elevation changes, aeolian sand transport measurements, bathymetric surveys and shoreline evolution assessments were used to investigate annual and seasonal patterns of dune development on Sfântu Gheorghe beach, the Danube delta coast, from 1997 to 2004. Dune volume increased consistently (1.96 m³ m^− 1 y^− 1 to 5.1 m³ m^− 1 y^− 1) over this 7-year period with higher rates in the southward (downdrift) direction. Dune aggradation is periodically limited by storms, each of which marks a new evolutionary phase of the beach–dune system. As a consequence of the variable beach morphology and vegetation density during a year, foredune growth occurs during the April–December interval while between December and April a slightly erosive tendency is present. The pattern of erosion and deposition shown by the topographical surveys is in good agreement with the sand transport measurements and demonstrates the presence of a vigorous sand flux over the foredunes which is 20–50% smaller than on the beach. This high sand flux, due to low precipitation and sparse vegetation cover, creates an aerodynamically efficient morphology on the seaward dune slope. The seaward dune face accretes during low to medium onshore winds (5.5–12 m s^− 1) and erodes during high winds (> 12 m s^− 1).     

Floodplain sedimentation in the Upper Mississippi Valley: Natural versus human accelerated

Understanding the time scales and pathways for response and recovery of rivers and floodplains to episodic changes in erosion and sedimentation has been a long standing issue in fluvial geomorphology. Floodplains are an important component of watershed systems because they affect downstream storage and delivery of overbank flood waters, and they also serve as sources and temporary sinks for sediments and toxic substances delivered by river systems. Here, ¹⁴C and ¹³⁷Cs isotopic dating methods are used along with ages of culturally related phenomena associated with mining and agriculture to determine rates of sedimentation and morphologic change for a reach of the upper Mississippi River and adjacent tributaries in southwestern Wisconsin and northwestern Illinois. The most important environmental change that influenced fluvial activity in this region during last 10,000 years involved the conversion of a late Holocene mosaic of prairie and forest to a landscape dominated by cropland and pastureland associated with Euro-American settlement. Results presented herein for the Upper Mississippi Valley (UMV) show that the shift from pre-agriculture, natural land cover to landscape dominance by agricultural land use of the last 175–200 years typically increased rates and magnitudes of floodplain sedimentation by at least an order of magnitude. Accelerated overbank flooding led to increased bank heights on tributary streams and, in turn, contributed to more frequent deep flows of high energy. These high energy flows subsequently promoted bank erosion and lateral channel migration, and the formation of a historical meander belt whose alluvial surface constitutes a new historical floodplain inset against the earlier historical floodplain. The new historical floodplain serves as a “flume-like” channel that provides efficient downstream transport of water and sediment associated with moderate and large magnitude floods. Floodplains on lower tributaries, however, continue to experience rates of overbank sedimentation that are of anomalously high magnitude given improved land cover and land conservation since about 1950. This lower valley anomaly is explained by minimal development of historical (agriculture period) meander belts because of relatively low stream power in these channel and floodplain reaches of relatively low gradient. In general, long-term pre-agriculture rates of vertical accretion between about 10,000 and 200 years ago averaged about 0.2 mm yr^− 1 in tributary watersheds smaller than about 700 km² and about 0.9 mm yr^− 1 on the floodplain of the upper Mississippi River where the contributing watershed area increases to about 170,000 km². On the other hand, rates of historical vertical accretion during the period of agricultural dominance of the last 200 years average between 2 and 20 mm yr^− 1, with short episodes of even higher rates during times of particularly poor land conservation practices. Significant hydrologic effects of mining and agricultural started by the 1820s and became widespread in the study region by the mid-19th century. The hydrologic and geomorphic influences of mining were relatively minor compared to those related to agriculture. High resolution dating of floodplain vertical accretion deposits shows that large floods have frequently provided major increments of sedimentation on floodplains of tributaries and the main valley upper Mississippi River. The relative importance of large floods as contributors to floodplain vertical accretion is noteworthy because global atmospheric circulation models indicate that the main channel upper Mississippi River should experience increased frequencies of extreme hydrologic events, including large floods, with anticipated continued global warming. Instrumental and stratigraphic records show that, coincident with global warming, a shift to more frequent large floods occurred since 1950 on the upper Mississippi River, and these floods generally contributed high magnitudes of floodplain sedimentation.