Baseflow Restoration in Minnehaha Creek Watershed with Stormwater Infiltration

By: Ryan Birkemeier, Water Resources Science graduate student in Department of Bioproducts and Bioengineering

Roughly three years ago, John Nieber of the Department of Bioproducts and Bioengineering and John Gulliver of the Department of Civil Engineering submitted a grant request to the Minnehaha Creek Watershed District (MCWD) and the Mississippi Watershed Management Organization (MWMO) to investigate the lack of water in dry years throughout Minnehaha Creek. Minnehaha Creek is arguably one of the most valued surface water features in the Minneapolis, MN metro area and is heavily used for recreation during the spring, summer, and fall. Flow in Minnehaha Creek is heavily dependent on discharge from the stream’s origin, Lake Minnetonka, the outlet of which is closed during most late summer periods to maintain water elevations in the lake resulting in low- (or no-) flow conditions in the creek. In addition, stormwater runoff entering directly to the creek from the creek’s largely urbanized watershed exacerbates extremes in flow conditions. As a result of these issues, there was interest in enhancing the cultural and ecosystem services provided by Minnehaha Creek through improvements in streamflow regime by reducing flashiness and sustaining increased low-flows.

The grant request to investigate low-flow conditions within Minnehaha Creek was accepted by the MCWD and the MWMO and work began in the spring of 2012. A post-doctoral researcher at the time, Trisha Moore (now Assistant Professor at Kansas State University), was charged with the task of identifying ‘losing’ areas (net flow of surface water to groundwater) and ‘gaining’ areas (net flow of groundwater to surface water) along the creek. This involved identification and quantification of the current sources of water contributing to low-flows in the creek through both field investigations and desktop analyses. Work on this source identification involved a number of different approaches, including analyses of the streamflow record using a hydrologic system model framework, examination of the underlying geology of the region, estimation of groundwater-surface water exchange rates within the channel and riparian corridor using temperature probe, seepage meter, and piezometer measurements, and analyses of the stable isotopes of oxygen and hydrogen in samples of stream water, groundwater, and rainfall. Due to this large workload, several undergraduate students were involved throughout the course of the project to help with fieldwork tasks and Water Resources Science graduate student, Ryan Birkemeier was added to the team in July of 2013 to expand upon and support existing work.

A draft report of results was submitted to MCWD and MWMO in December of 2013. The most recent results indicate that only a small portion of the catchment, probably the riparian zone, contributes to baseflows in Minnehaha Creek. These results appear to be supported by the observation that the low-vertical permeability limestone/shale bedrock layer underlying the surficial aquifer has been eroded away in past geological events in about 9% of the watershed, leading to a bedrock valley located in the chain-of-lakes area near Lake Calhoun and Lake Harriet. This eroded area essentially leaves the surficial aquifer ‘bottomless’ in that area and highly susceptible to vertical (downward) water loss. In addition, the analysis of the stable isotopes indicates that much of the low flow volume originates from surface storage including wetlands and small lakes within the watershed, rather than a groundwater source. The isotope analysis indicates that only about 5% of the surficial aquifer recharge water actually makes it to the creek; the rest is apparently lost to deep seepage. The groundwater-surface water exchange measurements along the main channel throughout the watershed show a ‘gaining’ trend in the upper reaches and a ‘losing’ trend in the lower reaches. To address the issue of low groundwater contribution to low-flows in the creek it has been proposed to divert stormwater to key locations within the riparian zone along the creek, and to infiltrate that water and store it for slow release to the creek during dry or no-precipitation periods. Work will continue over the upcoming months to investigate and provide solutions for low-flow conditions within Minnehaha Creek. An eventual goal is maintenance of flows throughout the watershed to improve recreational opportunities and stream habitat.

Joe Magner (Department of Bioproducts and Biosystems Engineering), Trisha Moore (Kansas State University) also contributed to this project.

For more information, visit St. Anthony Falls Laboratory Stormwater Research