Legacy Sediment Removal Project

Removing Streambed Sediment Leads to Cleaner Lakes Faster

algae bloom on Lake Kegonsa 2012
Algae bloom on Lake Kegonsa

Over the last few decades, the phosphorus concentrations of the Yahara chain of lakes (Mendota, Monona, Waubesa, Kegonsa) have led to an increase in the frequency and extent of algae blooms, creating green, slimy lakes and beaches.  Major sources of phosphorus have been identified as farm manure and fertilizer attached to sediments which are then transported to streams and lakes through rainfall runoff.  For over 30 years, the Dane County Land & Water Resources Department has assisted agricultural producers with conservation practice implementation to prevent soil erosion/barnyard runoff -- this is an essential step towards preventing additional phosphorus from entering the streams and lakes.  However, the question remained as to why algae blooms were still occurring despite decades of conservation work.

Breakthrough research in 2014 by Dane County in collaboration with the Wisconsin Department of Natural Resources (WDNR), discovered that phosphorus concentrations in the stream sediments of Dorn Creek (northeast of Lake Mendota) are seven times greater than in nearby crop fields.  Much of the stream sediment is known as legacy sediment, deposits formed by erosion from farm fields accumulating over decades of farming.  In the case of Dorn Creek, this sediment has existed since the late 1800s.  For Lake Mendota, if the accrued legacy sediment remains at the bottom of the streams, it was calculated that it would take almost 100 years to see a 50 percent reduction in phosphorus, a goal long-cited for lake clean up.

Dorn Creek Legacy Sediment
Streambed sediments in Dorn Creek

In order to reduce phosphorus concentrations and improve water quality decades sooner than 100 years, Dane County developed an innovative project to remove the phosphorus-laden sediment from the streambeds within the Yahara watershed.  The segments were selected in consultation with WDNR based on which segments had a large amount of legacy sediment (see map below). 


Because of this breakthrough, we will see clean lakes in our lifetime.

Joe Parisi, Dane County Executive

Removal of sediment (more than 2 feet thick in some stream sections) is expected to expose the original gravel stream beds allowing regeneration of fish populations that are now absent. Sediment removal (vacuumed from streambeds) is planned to begin Fall 2017.  Water quality will be monitored throughout the project and beyond.  Overall the goal is to achieve cleaner lakes faster.

Project Timeline

The project will be completed in two phases.  Phase 1 involves the assessment of the stream and sediments in the proposed locations (see map below).  The assessment consists of an evaluation of habitat health and laboratory analyses of sediments.  Habitat health provides information on existing conditions in order to ensure future sediment removal does not degrade the health of the aquatic system.  Laboratory analysis is performed to determine if existing phosphorus amounts in the stream sediments will improve water quality by their removal.  Also, the sediments are analyzed for contamination (e.g. heavy metals, PCBs) to determine the appropriate sediment removal techniques to maintain the health of the aquatic system.

Depending on the results of Phase 1, the site may be selected for Phase 2.  Phase 2 involves the creation of construction documents, submittal of permits, acquisition of construction easements to perform removal of stream sediments, and the removal of stream sediments awarded through a competitive bid process.


Proposed Phase 1 (Assessment) Timeline
Click on map to right for project status.

2017 Dorn Creek
2018 Token Creek, Lower Sixmile Creek
2019 Upper Dorn Creek, Upper Sixmile Creek, Yahara River
2020 Door Creek, Nine Springs Creek
Legacy Sediment Segments
Project Status (click to enlarge)

Related Links