Submitted to: Hydrological Processes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/6/2007
Publication Date: 9/17/2007
Citation: Salant, N., Hassan, M., Alonso, C.V. 2008. Suspended Sediment Dynamics at High and Low Flows in Two Small Watersheds. Hydrological Processes, 22: 1573-1587. Interpretive Summary: Suspended sediment concentrations (SSC) during flood events depend primarily on sediment supply from internal, in-channel sources or external, non-channel sources. Supply is usually greatest in the early period of a flood event, as readily available sediment sources – typically from outside the channel – are mobilized and rapidly transported. Under these conditions, small increases in discharge produce rapid increases in SSC. As these sources are depleted, however, SSC will rapidly decline, often long before a decrease in discharge. Because the bulk of total sediment load is mobilized during high flow events, suspended sediment loads are typically sampled within these periods to examine changes in the relationship between discharge and suspended sediment. Low flows that precede and follow flood events, on the other hand, have received comparatively little attention, due to the fact that during these periods hillslope and bank supplies are no longer accessible, fine sediment is supplied only from the bed, and SSC is assumed to be minimal. Typically, it is assumed that the amount of sediment mobilized during the low flows before and after flood events depends on discharge, not supply, such that SSC is directly related to the capacity of the flow. These assumptions have never been tested, however, despite the fact that bed storage may be an important source of fine sediment. In some systems, in-stream supply and depletion may be significant enough to change the discharge-SSC relationship during low flows. Furthermore, due to the long duration of low flow periods, low flows may be an important sediment supply mechanism that has heretofore been neglected. The present study tested these assumptions and provided evidence that bed composition and instream sediment supplies may play important roles in the mobilization and transport of fine sediment, especially at low discharges. In addition, the analysis of low-flow conditions, an approach unique to this study, provided insight into alternative and potentially significant factors that control the supply and transport of fine sediment. This knowledge will contribute to the development of improved predictors of fine suspended sediment loads that are a major pollutant of aquatic systems.
Technical Abstract: A long-term (nearly 20-yr) record of suspended sediment and discharge measurements on two reaches of an agricultural watershed are used to assess the influence of in-stream sediment supplies and bed composition on suspended sediment concentrations (SSC). We analyse discharge-SSC relationships from two small streams of similar hydrology, climate, and land use but widely different bed compositions (one dominated by sand, the other by gravel). Given that sand-dominated systems have more fine sediment available for transport, we use bed composition and the relative proportion of surface sand and gravel as representative of in-stream sediment supply. Both high flow events and low flow periods were analysed in order to distinguish external from in-stream sources of sediment and to assess the relationship between low flows and sediment supply. We find that SSC during low flows is affected by changes to sediment supply, not just discharge capacity, indicated by variation in the discharge-SSC relationship both within and between low flow periods. Results also demonstrate that suspended sediment and discharge dynamics differ between reaches; it appears that high bed sand fractions provide a steady supply of sediment that is quickly replenished, resulting in more frequent sediment-mobilizing low flow periods and relatively constant SSC between floods and years. In contrast, SSC of a gravel-dominated reach varies widely between flow events, with high SSC generally associated with only one or two high flow events. These results lend support to the idea that fine sediment is both more available and more easily transported from sand-dominated streambeds, especially during low flows, providing evidence that bed composition and in-stream sediment supplies may play important roles in the mobilization and transport of fine sediment. In addition, the analysis of low-flow conditions, an approach unique to this study, provides insight into alternative and potentially significant factors that control the supply and transport of fine sediment.