Do we know how much fluvial sediment reaches the sea? Decreased river monitoring of U.S. coastal rivers

Abstract

Given the present and future changing climate and human changes to land use and river control, river sediment fluxes to coastal systems are changing and will continue to change in the future. To delineate these changes and their effects, it is increasingly important to document the fluxes of river-borne sediment discharged to the sea. Unfortunately, broad-scale river sediment monitoring programs established more than 50 years ago in the United States have diminished substantially and now focus principally on the largest rivers and estuaries. Unless addressed, these data gaps will provide significant challenges in addressing fundamental scientific and management questions about the effects of climate change and sea-level rise in our estuaries and on our coasts.Water quality sampling has been conducted on American rivers for well over 100 years. Early sampling of water quality was in large part done to certify the potability of local drinking waters (e.g., Dole & Stabler, 1909), and suspended-sediment concentrations were measured in addition to dissolved and particulate species. These measurements allowed for the calculation of suspended-sediment discharge and thereby led to estimates of landscape denudation (e.g., Dole & Stabler, 1909). Expansion of river suspended-sediment sampling in the mid and later parts of the 20th century allowed scientists and planners to estimate the effects of deforestation, farming, construction, urbanization, and dam building on the fluxes of sediment (e.g., Curtis,

Evaluating suspended sediment and turbidity reduction projects in a glacially conditioned catchment through dynamic regression and fluvial process‐based modelling

“…The unresolved uncertainties described above highlight the need for greater investments in continuous SSC/T n monitoring, especially at locations both upstream and downstream of proposed BMPs, so that sufficient pre‐ and post‐project data can be used to unambiguously assess project performance. Current downward trends in SS monitoring programs across the United States (Warrick & Milliman, 2018) are in direct conflict with this need, and reflect poor planning when considering the cost of BMP installation and the utility of assessing project efficiency before committing to further restoration using the same technologies. NYC DEP and USGS have invested in such monitoring but unfortunately the extensive longitudinal inter‐basin monitoring in the study area started in 2016, after many of the STRPs were constructed.…”

“…BMP evaluation is further complicated by a frequent lack of T n or SSC monitoring data, some of which can be related to declines in the number of SS monitoring programs in the United States (Warrick & Milliman, 2018). Often, BMPs are installed in reaction to disturbance events and not as part of a long-term turbidity reduction strategy.…”

Toward Improved Accuracy of Remote Sensing Approaches for Quantifying Suspended Sediment: Implications for Suspended‐Sediment Monitoring

“…For the many global rivers with sparse or no monitoring, satellite-derived measurements may provide critical insight about patterns and trends in SSC that cannot be obtained without historical and/or spatially extensive data. These techniques offer the potential to supplement with current data monitoring programs that have been recently shuttered, particularly in the United States and Canada (Figure 12) (Warrick & Milliman, 2018), and to provide historical context for incipient monitoring programs elsewhere.…”

“…The geospatial focus of our remote sensing estimates of SSC comes at a time of transition for global monitoring of river material transport. Long‐standing monitoring programs in the United States have recently been terminated due to budget or logistical constraints (Lee & Glysson, 2013; Warrick & Milliman, 2018), and much of the observational data that are collected outside of the United States are held privately or are decentralized and difficult to access. Further, the synthesis of different data sets is challenging, as the methods by which agencies and private groups collect suspended sediment vary widely around the world.…”

“…Our lead/lag limit of +/− two days is more conservative than some other studies (Dethier et al, 2019;Montanher et al, 2014) and adequate for large rivers that do not change as rapidly as smaller, more flashy Figure 12. Suspended-sediment in situ monitoring at sites suitable for in situ-Landsat SSC calibration (river width greater than ~90 m) has declined from a peak in the 1980s-1990s in the United States (USGS) and Canada (WSC), both as reflected in number of stations (upper panel), and total number of samples taken (see also Warrick & Milliman, 2018). In particular, the number of stations making few measurements per year (<50) has declined dramatically, leading to a less cosmopolitan potential calibration data set.…”