ESTIMATING THE NUTRIENT AND SEDIMENT LOADING OF THE NORTH AND SOUTH BRANCH OF VALLEY CREEK BY USING FLUX

Erin Mortenson, St. Croix Watershed Research Station, Science Museum of Minnesota
Allison Stephens, College of St. Benedict, St. Joseph, Minnesota

Valley Creek, a high-quality trout stream in the Twin Cities metropolitan area, is a St. Croix River tributary located in southeastern Washington County. Monitoring of this valuable resource will help identify changes and pressures on the ecosystem as demand for development in the area increases. Valley Creek has two main branches: the Lake Edith-sourced north branch and the spring-fed south branch. The St. Croix Watershed Research Station has been monitoring both branches of this creek since late 1997.

The objective of this study was to calculate annual loads of sediment and nutrients for both branches during the period 1998-2004, based on available flow and water quality data. Flows were calculated from hourly stage records and stage-discharge relations. Storm samples for water quality analysis were collected by automated samplers and baseflow samples were collected bi-weekly from 1998-2000, and every other month thereafter. Analyzed variables included total and dissolved nitrogen and phosphorus (TN, DN, TP, DP), total suspended solids (TSS), and nitrate (NO3).

FLUX, a DOS-based computer modeling program designed for the Army Corps of Engineers and employed by Metropolitan Council Environmental Services, was used to calculate loads and flow-weighted concentrations of TN, DN, TP, DP, TSS, and NO3 in Valley Creek. FLUX modeling of TN, DN, NO3, and DP was successful, based on consistency with load results obtained from alternative calculation methods. The load and concentration of nitrates, the major component of TN in the system, was considerable in Valley Creek. A six-year (1999-2004) average of nitrate yield in the north branch was almost 700 kg/km2, and almost 3,000 kg/km2 in the south branch. The significant load of nitrates in this system is attributed to the high concentration of nitrates in groundwater in the Valley Creek watershed.

The FLUX modeling program seemed to have difficulty estimating, and tended to overestimate, TSS and TP loads. Again, this determination was based on load results obtained from alternative calculation methods. The average flow-weighted concentration of TP in Valley Creek was about 65 ppb, which exceeds the St. Croix Basin Water Resources Planning Team goal of 40 ppb TP in Lake St. Croix, the receiving water for Valley Creek. It will be beneficial to determine if and how FLUX overestimated the concentration of TP in Valley Creek to decide how much of an impact Valley Creek will have on TP levels in Lake St. Croix. FLUX may or may not be the ideal tool for estimating TSS and TP loads and concentrations in Valley Creek, but further study of its use is needed before determining an answer to this question. When this program produces reliable results, FLUX is a quick and easy tool for estimating stream loads and concentrations.