On big rivers like the Mississippi, the infrastructure of modern civilization – dams, locks, dikes, power plants, cities – has made life easier for people, but harder for fish and other denizens of the river. Restoration is a tricky problem. Economic reliance on these big rivers makes fundamental reversals like dam removals unlikely. Conservation laws and projects tend to be local, on the city or state level, and the river crosses many borders, complicating the restoration picture. Brenda Pracheil and colleagues at the University of Wisconsin, Madison, say tributaries have under-appreciated potential to compensate for habitat loss on the major concourses of the Mississippi Basin. The Platte, for example, has 577 kilometers of free-flowing, relatively intact habitat. It feeds into the heavily altered Missouri, a large mainstem river in the Mississippi Basin, and harbors many of the same fishes.
Pracheil found a correspondence between the volume rate of water flow and the presence of 68 large-river fishes, including paddlefish, blue catfish, and silver chub, most of which are threatened. A steep threshold separates tributaries with large-river fish from those without; 166 cubic meters per second is big enough for roughly 80% of large river specialist species. Below the threshold, almost none of these species are around. Pracheil says this threshold could be used to target tributaries for conservation attention. Existing regulatory structures don't allow improvements on tributaries to count toward mainstem restoration mandates. The UW scientists argue that more flexibility could, in some cases, provide a better return on investment of conservation dollars, complementing efforts on the larger rivers downstream.