St. Croix River Tributary Load Investigation (USGS 9KH60) - NOR_04_CMP13B


This project will monitor streamflow and nutrient concentrations as well as compute nutrient loads at 3 major tributaries to Lake St. Croix; Apple, Kinnickinnic and Willow Rivers. This information will be used in the implementation of the Lake St. Croix and Lake Mallaieu TMDLs, used to validate SWAT models of the Willow and St. Croix watersheds, used as input for a previously funded NPS/USGS intensive study, and used to measure the watershed response to agricultural land management practices initiated by the Discovery Farms project, sponsored by the University of Wisconsin, for the Willow River Basin.


Gauge stations will be funded on the Apple River near Sommerset (WBIC 2614000), the Kinnickinnic River near River Falls (WBIC 2601800) and the Willow River at Willow River State Park near Burkhardt (WBIC 2606900). Continuous flow data will be collected at each station and chemistry data collected monthly and during 10 events at each station. These samples will be collected by hand (monthly samples) and by automated samplers for the 10 event samples.


This project will quantify daily sediment and nutrient loads for three tributaries to Lake St. Croix during base flow conditions and storm events using the Graphical Constituent Loading Analysis System. These three tributaries are three of the six largest nutrient contributors to the St. Croix River. These data will be used to compute accurate loads on both annual and daily time frames to allow for validation of multiple components of the water and nutrient budgets in the SWAT Model.

Study Design

The USGS will install real time equipment to measure discharge and to sample river water during storm events in order to compute loads as follows: 1) Discharge will be measured over the range of flows that occur during the study using standard methods. A relation between stage and discharge will be refined from existing ratings for the sites as contemporary measurements are added.2) Nutrient samples will be collected at a fixed interval with additional automated samples during events. Fixed samples will be collected manually using the equal-width-increment (EWI) sampling method. Samples will be analyzed for suspended sediment in additon to nitrite, nitrate+nitrite, ammonia, total nitrogen, total phosphorus, and dissolved ortho-phosphate. Nutrient samples will be analyzed at the National Water Quality Lab and suspended sediment concentrations will be analyzed by the Kentucky Sediment Lab.3) A datalogger program that controls the gauge will be written to include operationg instructions for an ISCO water sampler. Samples will be collected at a regular interval during a storm event.4) Daily sediment and nutrient loads will be computed using GCLAS, the Graphical Constituent Loading Analysis System by which concentration are interpolated between samples and multiplied by daily stream flows.

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