The Mississippi River flows through the heart of Minneapolis, defining the city’s geography and history. The question of the river’s depth in this urban stretch is complicated because the river is far from a natural waterway. Within the city limits, the river’s profile is a series of controlled pools and dramatic drops, making the depth highly variable. The area surrounding the former St. Anthony Falls is heavily managed, meaning its depth is engineered rather than purely natural.
Measured Depth: Average and Extremes
The average depth of the Mississippi River in the Minneapolis area varies drastically between the channel and the shallower edges. The United States Army Corps of Engineers (USACE) maintains the Upper Mississippi River for commercial traffic, ensuring a minimum depth of nine feet in the navigation channel. This nine-foot depth is the standard maintained for the river’s commercial artery.
Away from the dredged channel, the river can quickly become much shallower, dropping to only a few feet or less in backwater areas and near shorelines. The deepest parts of the river are found in the pools created by the dam system. While the average depth of the river between St. Paul and St. Louis is often cited between nine and twelve feet, the maximum depth in the Minneapolis pools can be significantly greater.
The deepest points occur in the scour pools immediately downstream of dams and other structures. These areas, where water plunges over or through the dam, erode the riverbed to create deep depressions. In the Minneapolis section, maximum depths can reach approximately 30 to 40 feet in the deepest parts of the pools behind the dams, far exceeding the navigable channel’s minimum depth.
Controlling the Flow: Locks and Dams
The river’s depth through Minneapolis is not a natural feature but is instead a consequence of the man-made system of locks and dams. This system was authorized as part of the Upper Mississippi River 9-Foot Navigation Project to facilitate commercial barge traffic inland. The structures create a series of “slack-water” pools, which are stretches of deeper, slower-moving water upstream of each dam.
The Minneapolis stretch includes the former Upper St. Anthony Falls Lock and Dam and Lock and Dam No. 1 downstream, which create distinct pools. The Upper St. Anthony Falls Lock, which is now permanently closed, was notable for having the highest lift of any lock on the Mississippi River, with a drop of 49 feet. This structure, along with the other dams, is operated to maintain the required water elevation in the pool above it, ensuring the nine-foot channel depth is consistently met.
The dam structures themselves are a series of concrete piers with movable gates, which allows the Corps of Engineers to regulate the pool elevation. These gates are adjusted to hold the water back, artificially raising the water level upstream to form the deep navigation pools. The primary function of these dams in Minneapolis is for navigation and hydroelectric power generation, not for flood control, meaning they are operated to maintain a specific pool height.
Natural Variations in Water Level
While the dams establish the base water level, the actual depth still fluctuates due to natural, dynamic factors. The river’s gauge height, which is the measured water level, is directly influenced by the volume of water flowing into the system, measured in cubic feet per second (cfs). Seasonal changes are the main driver of these depth variations.
Spring snowmelt and heavy regional precipitation lead to high flow events, causing the river level to rise and the current to accelerate. Conversely, periods of summer drought or low precipitation result in lower flow rates, causing the pool elevation to drop slightly and exposing shallower areas. The Corps of Engineers constantly adjusts the dam gates to manage these fluctuations, attempting to keep the pool elevation stable for navigation, but the river’s depth remains responsive to the weather upstream.
Navigating the River: Depth and Safety Considerations
The highly variable depth profile of the Mississippi River in Minneapolis presents distinct considerations for public safety and navigation. Recreational boaters must remain within the marked navigation channel to avoid running aground, as the depth drops off quickly in the areas outside of the maintained nine-foot channel. Shallow areas can conceal submerged hazards like wing dams, rocks, and uprooted trees, which pose a significant threat to boat hulls and propellers.
The deep pools and the presence of dam structures create powerful, hidden dangers for anyone in or near the water. The currents, especially in the tailwaters immediately downstream of the dams, are extremely strong and fast, creating unpredictable eddies and undertows. Swimming or wading near these structures is strongly discouraged because the force of the water and the sudden drop-offs into the deep scour pools can be fatal. Furthermore, the river’s high flow can carry large amounts of debris, including submerged logs and other hazards, making any activity in the water risky.