From Rainfall Patterns to Streamflow Hydrodynamics and Lake Water Quality

Stream event flows generated by urban watershed stormwater runoff can have a major impact on nutrient and sediment loads into lakes and reservoirs.   Stream flows are intensely monitored to better estimate their volume and nutrient loadings.  Much effort is also spent in getting a better picture of the many in-lake processes that ultimately control the lake’s internal nutrient loading.  However, the actual physical movement and flow path of the inflow into a lake is seldom investigated or considered as a culprit in deteriorating water quality.  Once an inflow reaches a lake, it does not randomly mix with the lake water nor do all inflows mix in the same way.  Inflows, depending on their physical characteristics (density, temperature) will seek areas of the lake profile that have similar characteristics.  These intrusions can deliver nutrients at varying depths that may promote algae growth directly at the surface or enhance nutrient delivery into the surface.  These underflows may also deliver sediment or dissolved oxygen of various concentrations and disrupt seasonal conditions or simply sink to the bottom and create various degrees of water column mixing conditions which will be discussed in this presentation.  We will then share results of our field data and 3D modeling efforts that suggest that flow hydrodynamics play a major role in forming the water quality condition in a shallow bay.  The 3D model, a coupled 3D hydrodynamic and ecological model, was successfully applied to number of bays of a morphologically complex lake (Lake Minnetonka, MN, USA).  The model was able to capture water quality variations including significant sudden (days) water quality changes caused by stream inflow events that would not have been otherwise detected by typical summer monitoring schemes.  Model simulations, confirmed by field data, indicate that changes in total phosphorus and dissolved oxygen concentrations in deeper water columns were more likely the consequences of physical perturbations caused by storm events flows.  The model was also used to examine and compare the coolwater fish habitat under two different climate conditions.  Our research shows that plunging storm inflows can drastically change the in-lake water temperature, dissolved oxygen, and total phosphorus concentrations.  Monitoring programs need to incorporate necessary components and time scales to capture these changes.  The goal is to encourage lake managers, water resources engineers, and impaired water body/TMDL managers to begin considering flow hydraulics as a significant component for managing their water resources.