Background

Lake Pepin is a naturally occurring lake on the Mississippi River, spanning the 26-mile stretch from Red Wing to the Chippewa River delta. With an average width of 1.7 miles, it is the widest stretch of the entire Mississippi and covers nearly 30,000 acres.  It receives drainage from four major watersheds (Minnesota River Basin, Cannon River Basin, St. Croix River Basin, and Upper Mississippi River Basin), encompassing almost half of Minnesota and parts of South Dakota, Iowa, and Wisconsin. At the end of the last glacial period, this unique natural impoundment extended even further upriver to present-day St. Paul, but sediments flowing in from upstream have been gradually decreasing the size of the lake over time. The lake is a treasured and vital natural resource, home to hundreds of aquatic plant and animal species and a globally important flyway for migratory birds. It has also become the area’s economic backbone, fueling tourism and recreation industries that support communities along both sides of the lake. Click the video and read below to learn more.

Click this video for an overview of Lake Pepin and how water quality impairments affect surrounding communities. The cover photo shows the high sediment load from the Minnesota River flowing into the Mississippi River, just upstream from Lake Pepin.

Sedimentation & Eutrophication

Map of Upper Lake Pepin that compares depths from 1890-1990. The light blue areas depict depths that have fallen below 3 feet since 1890. the darker blue areas show stretches still greater than 3 feet deep, which includes the navigation channel dredges for transportation. Since this map was created, another 27 years of sediment has accumulated thereby extending the light blue regions even further.

Map of Upper Lake Pepin that compares depths from 1890-1990. The light blue areas depict depths that have fallen below 3 feet since 1890. the darker blue areas show stretches still greater than 3 feet deep, which includes the navigation channel dredges for transportation. Since this map was created, another 27 years of sediment has accumulated thereby extending the light blue regions even further.

Although sedimentation is a normal geological process, scientists have concluded that the rate of sediment supply  to Lake Pepin has increased more than tenfold since the arrival of European settlers. It is now anticipated that the upper third of the lake will be unsuitable for recreation by the end of this century and the entire lake substantially filled within 340 years.  Without the human-induced increase in sedimentation rates, Lake Pepin would be one meter deeper and expected to provide recreational and commercial value for another 4,000 years. 

 A reduction in sediment loading to Lake Pepin is critical long-term, and must be a priority. Current sedimentation rates threaten the ecological integrity and future of Lake Pepin, posing harm to aquatic and bird habitat, recreational use options, tourism, and local economies. Moreover, scientists have documented that the increased sedimentation is already creating adverse conditions for aquatic vegetation, thus threatening the fish and bird populations that depend upon lake vegetation for survival.

Lake Pepin is also threatened by excess nutrients leading to eutrophication and downstream impacts. Phosphorous is the limiting nutrient and excess input from the surrounding landscape has led to eutrophication, characterized by algae blooms that reduce light and oxygen thereby threatening animal and plant survival. It is estimated that the accumulation of phosphorus in Lake Pepin sediment has increased 15-fold since 1830. This eutrophication process is tightly linked with sedimentation because phosphorous binds to soil particles, which are transported together down the watersheds to Lake Pepin. Nitrogen input is also affecting local water quality with cascading impacts all the way to the Gulf of Mexico, where a large hypoxic area, or "dead zone", has formed at the Mississippi delta leading to environmental problems, such as fish kills.

The problem is driven by natural and human-induced processes throughout all of the major watersheds, which creates complexitiy as stakeholders collaborate on possible solutions. The Minnesota River Basin (MRB), however, is the main concern since it contributes 80-90% of the sediment accumulating in Lake Pepin. LPLA has partnered with government, academic, non-profit, and citizen stakeholders to raise awareness, increase scientific understanding, and implement projects to protect Lake Pepin. While most projects are directed at mitigating upstream sources of sediment and nutrients, LPLA is also leading management efforts to reverse current impacts already affecting the lake. 

Governance & Hydrological Complexity

Map showing Upper Mississippi River turbidity impairment contributing to downstream sedimentation in Lake Pepin (red) and Lake Pepin eutrophication impairment from excess nutrients (green). These waterbodies are assessed in separate TMDLs, but jointly affect the health of Lake Pepin.

Map showing Upper Mississippi River turbidity impairment contributing to downstream sedimentation in Lake Pepin (red) and Lake Pepin eutrophication impairment from excess nutrients (green). These waterbodies are assessed in separate TMDLs, but jointly affect the health of Lake Pepin.

Action is supported by science and policy at the state and federal level. Lake Pepin and many upstream waterways have been placed on the US EPA's Impaired Waters List, established by the federal Clean Water Act (CWA). Once listed, the CWA requires scientific assessments to determine how much of a pollutant can enter a waterbody without it exceeding water quality standards. These scientific assessments, referred to as TMDLs (Total Maximum Daily Loads), also identify pollution sources, allocate load reductions, and establish water quality improvement goals. They essentially serve as a diet plan for a specific waterbody exceeding water quality standards.

It seems straighforward, but the process is complicated by the fact that waterbodies-lakes, rivers, streams, and wetlands-are all connected. Moreover, turbidity and eutrophication impairments are highly inter-related problems because phosphorus binds to soil particles as they travel together through the watershed. As a result, multiple TMDLs relate to the health of Lake Pepin, including: The Lake Pepin Eutrohpication TMDL (in progress) and  the Upper Mississippi Turbidity TMDL. The later was completed in 2016 and requires the following sediment (TSS) reductions from contributing watersheds: 

  • 50% reduction from Minnesota and Cannon Rivers
  • 20% reduction from the Upper Mississippi River, St. Croix and Vermillion Rivers
  • 50% reduction in internal sediment loading within Lake Pepin

These sediment reduction goals, like many others outlined in state TMDLs, are highly difficult to achieve. It requires action and cooperation among diverse government, non-profit, academic, and citizen stakeholders. The MPCA must provide reasonable assurance that the load reductions are feasible, but thus far water quality progress is slow and goals are stretched out over long time scales. For example, The Le Sueur River WRAPS Report  sets forth a 65% reduction of TSS over a 65 year period.  Social barriers, such as jurisdictional overlap, limited project funding, low staff capacity, necessary technical expertise, and economic disincentives make achievement of such milestones and goals even more unlikely. 

Due to the governance and hydrological complexity of the problem, there is no quick or easily solution to Lake Pepin's water quality woes.  As such,upstream pollution and erosion mitigation is critical to long-term sustainability, but it is not an immediate panacea. Protecting Lake Pepin requires upstream mitigation and onsite water quality management via restoration.