Although sediment is a natural constituent of rivers, excess loading to rivers and streams is a leading cause of impairment and biodiversity loss. Remedial actions require identification of the sources and mechanisms of sediment supply. This task is complicated by the scale and complexity of large watersheds as well as changes in climate and land use that alter the drivers of sediment supply. Previous studies in Lake Pepin, a natural lake on the Mississippi River, indicate that sediment supply to the lake has increased 10-fold over the past 150 years. Herein we combine geochemical fingerprinting and a suite of geomorphic change detection techniques with a sediment mass balance for a tributary watershed to demonstrate that, although the sediment loading remains very large, the dominant source of sediment has shifted from agricultural soil erosion to accelerated erosion of stream banks and bluffs, driven by increased river discharge. Such hydrologic amplification of natural erosion processes calls for a new approach to watershed sediment modeling that explicitly accounts for channel and floodplain dynamics that amplify or dampen landscape processes. Further, this finding illustrates a new challenge in remediating nonpoint sediment pollution and indicates that management efforts must expand from soil erosion to factors contributing to increased water runoff.

Rivers in watersheds dominated by agriculture throughout the US are impaired by excess sediment, a significant portion of which comes from non‐field, near‐channel sources. Both land‐use and climate have been implicated in altering river flows and thereby increasing stream‐channel erosion and sediment loading. In the wetland‐rich landscapes of the upper Mississippi basin, 20th century crop conversions have led to an intensification of artificial drainage, which is now a critical component of modern agriculture. At the same time, much of the region has experienced increased annual rainfall. Uncertainty in separating these drivers of streamflow fuels debate between agricultural and environmental interests on responsibility and solutions for excess riverine sediment. To disentangle the effects of climate and land‐use, we compared changes in precipitation, crop conversions, and extent of drained depressional area in 21 Minnesota watersheds over the past 70 years. Watersheds with large land‐use changes had increases in seasonal and annual water yields of >50% since 1940. On average, changes in precipitation and crop evapotranspiration explained less than one‐half of the increase, with the remainder highly correlated with artificial drainage and loss of depressional areas. Rivers with increased flow have experienced channel widening of 10–40% highlighting a source of sediment seldom addressed by agricultural best management practices. Copyright © 2013 John Wiley & Sons, Ltd.

This study determined the importance of air−water exchange of toxaphene in the Great Lakes by comparing this flux to other inputs and outputs in a mass balance model. Our overall goal was to test the hypothesis that the current water concentrations of toxaphene in Lake Superior are due to physical limnological differences between it and the lower Great Lakes and secondarily to evaluate whether nonatmospheric inputs of toxaphene have had an impact on current toxaphene burdens in lakes Superior and Michigan. A series of water samples from the Great Lakes were analyzed for toxaphene. A static mass budget is presented for lakes Superior, Michigan, and Ontario. A dynamic model of toxaphene behavior in lakes Superior and Michigan from 1950 to 1995 is then presented. The results of this model support our hypothesis that the colder temperatures and lower sedimentation rates in Lake Superior are responsible for its high water concentrations of toxaphene and that there is no evidence of nonatmospheric sources of toxaphene to Lake Superior. However, the model supports the conclusion that there were nonatmospheric sources of toxaphene to Lake Michigan.

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