Changes in lateral floodplain connectivity accompanying stream channel evolution: Implications for sediment and nutrient budgets

Abstract

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.

Riparian Land-Use, Stream Morphology and Streambank Erosion within Grazed Pastures in Southern Iowa, USA: A Catchment-Wide Perspective

“…This dynamic process of channel modification is described by the channel evolution model [6], which is also important descriptors when addressing the variability in the sources of sediment load (top soil surface, streambank, stream bed, floodplain storage etc.) at the watershed scale and, in some cases, it may mask improvement in sediment reduction achieved by the edge of field practices [10]. Along with the effects of stream geomorphologic adjustment, riparian land-uses such as row-crop and grazed pasture have been shown to increase the bank soil loss and associated total-P by destabilizing the bank soil and by altering the flow regime of the stream itself [15,27,31].…”

“…The impact of local riparian land-use factors, such as animal grazing intensity, on bank erosion and/or cross-sectional channel modification has not been well established. This is partially due to the many interacting factors such as bank soil properties (cohesion of the bank soil or major textural unit), stream flow characteristics (number of high flow events and their frequencies), and channel morphology (stream bed slope and sinuosity; progression of channel evolution), all of which can play crucial roles in the adjustment of bank physiography and sediment loading [7][8][9][10]. However, some studies have concluded that riparian cattle grazing can initiate the first step towards greater eroded bank area and consequent destabilization [11,12] Indeed, a three-year study by Zaimes et al [13] recorded greater streambank erosion rates from grazed pastures (continuous, rotational, and intensive rotational) than from riparian forest buffers and grass filters.…”

Fine-Grained Sediment and Phosphorus Storage in a Suspended-Load-Dominated, Alluvial Channel

“…Thus, the Gunder possesses an inherent resistance to fluvial erosion. Results from Beck et al (2019) exemplify this resistance, documenting a relatively minor increase (+9.4%) in the distance from top bank to thalweg (i.e., channel downcutting) over a 17-year period (1998 to 2014). The Gunder is directly overlain by the Roberts Creek member (silty clay loam) (Bettis et al, 1992).…”

“…Mean channel gradient was determined to be 0.0017 m m -1 in 2017. Several stages of stream channel evolution (Simon, 1989) have been documented in Walnut Creek, with channel geomorphic surveys indicating stage IV (degradation and widening) as the most prevalent within the watershed (Schilling and Thompson, 2000;Schilling and Wolter, 2000;Beck et al, 2019). Walnut Creek's floodplain is comprised of a series of loess-derived Holocene alluvial deposits, collectively known as the DeForest Formation (Bettis, 1990).…”

Continuous watershed-scale hydrologic modeling of conservation practices for peak flow reduction

“…The transport of chemicals is also a dynamic problem to which hydrologic modeling can provide insight. Three recent papers from 2016, 2018, and 2019 have looked at the spatial and temporal patterns of the transport of polychlorinated biphenyls, metals, and nutrients during flooding (Alam & Dutta, 2016;Beck et al, 2019;Lu et al, 2016;Mansoor, Louie, Lima, Van Cappellen, & MacVicar, 2018). The use of hydrologic modeling to understand behavior and response to changes in hydrologic systems is an ongoing area of research with many different approaches and questions being answered.…”