Back in the 1800s, rivers were the highways of the times. But waterfalls – like St. Anthony Falls on the Mississippi river – provided significant obstacles for boats, their passengers, and their cargo. In Minnesota, the “twin” cities of Minneapolis and St. Paul weren’t initially equally prosperous. While St. Paul, down-river from the falls and its dangerous rapids grew into a commercial hub, Minneapolis was isolated. It wasn’t until the 1950s/60s that a couple of sets of locks were built at the falls, a massive engineering undertaking that allowed barges and boats to, at last, ascend beyond the falls and connect Minneapolis with the commerce of the Mississippi.


So, how does the system work? For a boat going downstream, a lock chamber is first filled by opening a filling valve. With the upper and lower gates closed, the level of the chamber rises to the upstream level. The upper gate then opens and the boat moves into the lock. To lower the boat, the gates are closed behind it, the filling valve is closed, and the emptying valve is opened. The pressure of the higher water in the lock drains to the downstream level. The lower gates are then opened and the boat moves out at the lower water level. For a boat going upstream, this sequence is reversed. The process of filling or emptying the lock chamber is powered entirely by gravity and controlled by valves.


In 2015, the St. Anthony Falls lock was closed down, not due to any kind of malfunction, but in order to halt the spread of invasive fish, mainly carp. As the carp uproot and kill aquatic plants (a food source for native fish and birds), they also upset the ecosystem. The sediment that gets stirred up in the process of uprooting increases phosphorus levels in the water (phosphorus loves to attach itself to soil particles). This is a critical nutrient for life but too much of it can lead to overgrowth of algae. The algae reduce the levels of dissolved oxygen in the water (a process called eutrophication), which can kill other aquatic species in the water. In worst-case scenarios, with the right combination of warmer water temperature and lots of sunlight, the algae can grow out of control, into large toxic blooms, which can also lead to severe illness, and even death in humans. Algal blooms are a sign of an ecosystem out-of-balance. It’s expected to get worse with climate change. While we can’t do much about natural causes for these blooms like river floods which mobilize nutrients, and the upwelling of nutrients from the seafloor following storms, we CAN control the flux of invasive species into ecosystems, as well as minimize other phosphorus-rich nutrient sources: runoff from fertilizer, sewage and livestock waste.



photo: U. Peña, Mississippi River, Minneapolis, Minnesota