Minnesota Water Resources Conference - Water Resources Center ...

BOOK OF ABSTRACTS
Tuesday, October 28
Concurrent Sessions IV 10:00–11:30
Track A: Lake Pepin TMDL
Why is Lake Pepin Filling-up at a Rapid Rate
Satish Gupta, University of Minnesota, gupta002@umn.edu; Heather Johnson, Minnesota Department of Agriculture; Greg
Spoden, Minnesota Department of Natural Resources
Based on core data from Lake Pepin, scientists at St. Croix laboratory have shown that sedimentation rates
have been increasing in recent years. Average lake filling rate varied from 1.15 cm yr -1 in mid 1970s to1.6 cm yr -1
in mid 1990s. In upper reaches of the Lake, the sedimentation rates were greater than 3 cm yr -1 over the last
three decades. The question is: why is Lake Pepin filling up at a rapid rate in recent years when flow adjusted
TSS concentration have been either decreasing or stable In this paper, we present the trend analysis in TSS
concentration in the Minnesota River using parametric and non-parametric statistical methods. Then we
show how recent wet climate (increased precipitation as well as frequency of larger storms) in the region have
contributed to increased flow and thus increased sediment loads in the Minnesota River. We believe these
increased sediment loads are the reasons for increased rate of sedimentation in Lake Pepin.
Water Quality Model for Support of the Upper Mississippi River—Lake Pepin TMDL: Results of Load
Reduction Scenarios
Joseph DePinto, LimnoTech, jdepinto@limno.com; Hans Holmberg, Limno Tech; Todd Redder, Limno Tech; Edward Verhamme,
Limno Tech; Norman Senjem, Minnesota Pollution Control Agency; Hafiz Munir, Minnesota Pollution Control Agency
Nationally, nutrients and sediment/siltation rank among the top four impairments reported by states on their
303(d) lists. The State of Minnesota is developing a TMDL for the Upper Mississippi River and its watershed
for both turbidity and nutrient enrichment (total phosphorus and chlorophyll a). This stretch of the river
receives both point and non-point source loads of solids and nutrients from an extremely large contributing
watershed (48,634 square miles, encompassing about 48% of the State of Minnesota and a portion of Wisconsin.
The impaired river reach contains two impoundments, Spring Lake and Lake Pepin, located behind two of
the four Lock & Dam control structures that bound the three morphometrically and hydraulically distinct
“pools” of the system. Under contract to the State, LimnoTech has developed, calibrated, and confirmed a
linked hydrodynamic-sediment transport-eutrophication model (the UMR-Lake Pepin model) to support
the computation of the TMDLs for this system. Subsequent to the calibration and confirmation, the model
has been applied to predict the response of the system to a series of load reduction scenarios that will aid
in computation of the TMDLs, and the allocation of allowed loads from various sources in order to most
efficiently meet the TMDLs. This presentation will illustrate the results of those load reduction scenarios and
discuss how those results will be applied to support the TMDL. Results of the model application suggest that
significant sediment and phosphorus load reductions will be required for major tributaries, accompanying the
phosphorus effluent reductions being implemented at wastewater treatment plants, will be required to achieve
both turbidity and chlorophyll a targets for the system.
59 Minnesota Water Resources Conference, October 27–28, 2008