Streambank erosion may represent a significant source of sediment and phosphorus (P) to overall watershed loads; however, watershed-scale quantification of contributions is rare. In addition, streambanks are often comprised of highly variable stratigraphic source materials (e.g., alluvial deposits), which may differentially impact in-channel P dynamics once eroded. The objective of this study was to quantify sediment and total phosphorus (TP) losses from four materials comprising streambanks within a 5218 ha watershed in Iowa, USA. Streambank-face surveys, erosion pins, and soil analyses were used to quantify surface area representation, recession, and losses of sediment and TP over a two-year period. Cumulative, whole-bank gross mean recession totaled 18.6 cm over two years, and material-specific gross mean recession ranged from 15.5 to 64.1 cm. Cumulative, whole-bank mean gross mass losses totaled 0.28 Mg sediment and 0.7 × 10 −5 Mg TP per meter channel length. Annual sediment losses equated to 4-44% of historic suspended sediment loads. Stratigraphy was significant in gross material erosion and losses, with lower materials (i.e., bank toe region) exhibiting the greatest recession rates and cumulative recession. Weathered/colluvial material dominated total bank face surface area (88.3%), and contributed the greatest proportion of sediment and TP mass loss (66, 68%, respectively) versus other streambank materials. recent work by Gellis et al. [13] suggests that in-channel material is the primary source of watershed suspended sediment. Research by Palmer et al. [14] also suggests streambanks as a significant source of Walnut Creek annual suspended sediment loads; however, high variability in annual contributions exists (0-53%). Global studies have documented similar ranges, with streambanks contributing between <19% [15][16][17], and up to 89% [18] of annual suspended sediment loads. Significant, yet highly variable, streambank contributions have also been documented for total phosphorus (TP) annual loads [8,19,20] within the USA and globally [18,21]. However, studies quantifying streambank SS and TP loading remain limited in both number and regional representation [4]. Because of the relative paucity and high variability of data, streambank SS and TP loading is commonly absent from local and regional water quality strategies aimed at reducing nutrient loading, such as the Iowa Nutrient Reduction Strategy (INRS) [22].Streambank material characteristics (e.g., bulk density, structure, texture) exhibit a high degree of variation at the individual bank and watershed scales [23][24][25][26], and banks in alluvial streams may be comprised of numerous, distinct, stratigraphic alluvial units [27,28]. Material variation among units, along with stratigraphic position, may have significant implications for sediment and P loading, as units may be impacted differently, both spatially and temporally, by specific erosional processes [29,30]. Inherent unit material characteristics (e.g., equilibrium P concentration, degree of P...

Floodplain storage commonly represents one of the largest sediment fluxes within sediment budgets. In watersheds responding to large scale disturbance, floodplain-channel lateral connectivity may change over time with progression of channel evolution and associated changes in channel geometry. In this study we investigated the effects of channel geometry change on floodplain inundation frequency and flux of suspended sediment (SS) and total phosphorus (TP) to floodplain storage within the 52.2 km2 Walnut Creek watershed (Iowa, USA) through a combination of 25 in-field channel cross section transects, hydraulic modeling (HEC-RAS), and stream gauging station-derived water quality and quantity data. Cross sectional area of the 25 in-field channel cross sections increased by a mean of 17% over the 16 year study period (1998-2014), and field data indicate a general trend of degradation and widening to be present along Walnut Creek's main stem. Estimated stream discharge required to generate lateral overbank flow increased 15%, and floodplain inundation volume decreased by 37% over study duration. Estimated annual fluxes of SS and TP to floodplain storage decreased by 61 and 62% over study duration, respectively. The estimated reductions in flux to floodplain storage have potential to increase watershed export of SS and TP by 9 and 18%, respectively. Increased contributions to SS and TP export may continue as channel evolution progresses and floodplain storage opportunities continue to decline. In addition to loss of storage, higher discharges confined to the channel may have greater stream power, resulting in further enhancement of SS and TP export through accelerated bed and bank erosion. These increased contributions to watershed loads may mask SS and TP reductions achieved through edge of field practices, thus making it critical that stage and progression of channel evolution be taken into consideration when addressing sediment and phosphorus loading at the watershed scale.

Enteric microbial pathogens originating from agricultural practices pose a serious threat to surface water quality and human health. A two-phase study was conducted in southern Illinois that examined the ability of various riparian buffer species to attenuate fecal coliform bacteria and Escherichia coli (E. coli) in overland flow, while also assessing E. coli persistence in associated buffer soils. Plots established with giant cane (Arundinaria gigantea (Walt.) Muhl.), Kentucky Bluegrass (Poa pratensis), Orchardgrass (Dactylis glomerata L.), and non-vegetated control buffers were subjected to artificial flooding with cattle (Bos taurus) manure-laden water. Mean bacteria concentrations in overland flow exiting all plots were significantly higher than concentrations found in the source slurry. To assess E. coli persistence in plots, overland flow was collected from a simulated rainfall event 4 days following slurry application, and from natural rainfall events 22, 46, 94, 97, and 111 days after the application. Overland flow exiting all plots (including non-vegetated controls) exhibited mean E. coli concentrations significantly higher than those found in the initial slurry applied. The observed concentrations may have been the result of overland flow characteristics and high levels of ambient fecal indicator bacteria (FIB) present with plots. The moist, shaded conditions created by plot vegetation may have produced prime habitat to harbor FIB. Vegetated buffer strips (VBS) may have been effective at attenuating FIB, however, performance may have been masked by the exceptionally high ambient FIB populations that were present in adjacent soils. The young age of the VBS (\1 year) may have also influenced their seemingly poor FIB attenuation. As the VBS age, increases in above and below ground vegetative material may act to increase VBS attenuation capabilities.

Orthophosphorus (OP) is the form of dissolved inorganic P that is commonly measured in groundwater studies, but the spatial distribution of groundwater OP across a watershed has rarely been assessed. In this study, we characterized spatial patterns of groundwater OP concentrations and loading rates within the 5218ha Walnut Creek watershed (Iowa) over a two-year period. Using a network of 24 shallow (<6m) monitoring wells established across watershed, OP concentrations ranged from <0.01 to 0.58mg/l in all samples (n=147) and averaged 0.084±0.107mg/l. Groundwater OP concentrations were higher in floodplains and OP mass loading rates were approximately three times higher than in uplands. We estimated that approximately 1231kg of OP is present in floodplain groundwater and 2869kg is present in upland groundwater within the shallow groundwater zone (0-5m depth). Assuming no new inputs of OP to shallow groundwater, we estimated it would take approximately eight years to flush out existing OP mass present in the system. Results suggest that conservation practices focused on reducing OP loading rates in floodplain areas may have a disproportionately large water quality benefit compared to upland areas.

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