Abstract
Large runoff, sediment, and nutrient exports from watersheds could occur due to individual extreme climate events or a combination of multiple hydrologic and meteorological conditions. Using high-frequency hydrologic, sediment, and turbidity data we show that freeze–thaw episodes followed by intense winter (February) rainstorms can export very high concentrations and loads of suspended sediment and particulate organic carbon (POC) and nitrogen (PN) from mid-Atlantic watersheds in the US. Peak suspended sediment (> 5000 mg L−1), POC (> 250 mg L−1) and PN (> 15 mg L−1) concentrations at our 12 and 79 ha forested watersheds for the February rainfall-runoff events were highest on record and the fluxes were comparable to those measured for tropical storms. Similar responses were observed for turbidity values (> 400 FNU) at larger USGS-monitored watersheds. Much of the sediments and particulate nutrients likely originated from erosion of stream bank sediments and/or channel storage. Currently, there is considerable uncertainty about the contribution of these sources to nonpoint source pollution, particularly, in watersheds with large legacy sediment deposits. Future climate projections indicate increased intensification of storm events and increased variability of winter temperatures. Freeze–thaw cycles coupled with winter rain events could increase erosion and transport of streambank sediments with detrimental consequences for water quality and health of downstream aquatic ecosystems.
Highlights
Sediment and sediment-bound nutrients and pollutants are among the leading causes of water quality and habitat impairment in our nation’s water bodies (USEPA 2016a)
To investigate the sources of the sediment associated with the winter 2016 storms, we evaluated previously collected (Johnson et al submitted; Rowland et al 2017) chemical fingerprint data for potential watershed sediment sources in a principal component mixing space
Notwithstanding the intensity, the rainfall occurrence itself was unprecedented, given that precipitation typically occurs as snow during this time of the year in this region
Summary
Sediment and sediment-bound nutrients and pollutants are among the leading causes of water quality and habitat impairment in our nation’s water bodies (USEPA 2016a). Sediment input to streams and aquatic ecosystems can originate from upland sources such as erosion of cropland, urban construction sites, and unpaved road surfaces, and/or from near-stream sources such as streambank and bed erosion (USEPA 2016b; Walling 2005). Identifying these disparate sediment sources and their relative contributions to watershed sediment yields is critical given that land management and mitigation strategies differ considerably for each sediment source (Wohl 2015). Yellen et al (2016) showed that very high antecedent 30-day precipitation coupled with rainfall from tropical storm Irene yielded the highest sediment exports recorded for the Connecticut River watershed in Massachusetts, USA. Understanding how a variety of climatic and hydrological conditions (storm intensity, seasonal storm timing, antecedent moisture conditions, etc.) could combine to yield large sediment and nutrient exports is an important challenge and knowledge gap (see Garner et al 2015; Naylor et al 2017)
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