STREAM TRANSPARENCY PROFILES IN AN AGRICULTURAL LANDSCAPE, EASTERN GREAT PLAINS, USA

Southwestern Minnesota and northwestern Iowa are an integral part of the classic Midwestern US Corn Belt. The Rock River and its tributaries drain a portion of this area where commodity row crops dominate the landscape. Starting in 2007, more than three dozen profiles have been constructed by plotting transparency tube measurements against distance along the stream. Transparency profiles for the mainstem of the Rock River and for tributaries including the Little Rock River and Kanaranzi Creek range from as few as four stations in 5 mi (8 km) to more than twenty stations over 60 mi (97 km). The profiles document stream transparency for a variety of spring runoff events, events related to specific rainstorms, and normal flow conditions over extended periods of time.

The vast majority of the transparency profiles show an overall decrease in transparency downstream. Departures from this prevailing downstream decrease are generally related to specific rain events or land use patterns. In addition, profiles with low overall transparencies tend to be fairly linear with small slope values and individual stream reaches are indistinguishable. In contrast, profiles with higher transparencies have larger slope values and in some cases are exponential; individual stream reaches are better defined in these high transparency profiles.

Transparency profiles are an effective way to summarize stream loads over the total length of a stream. They may prove to have useful applications in evaluating TMDL impairment and in assessing the influence of land use patterns on water quality. Ultimately, they may also aid in distinguishing the relative contribution of overland flow and channel bank erosion to stream loads.