Levee Setbacks and Meandering Rivers
By Irv Schiffman
For a variety of reasons River Partners is supportive of moving levees back from the river and we are involved in a number of levee setback projects. One of the reasons that we support levee setbacks is that it allows the river to follow its natural course of meandering across the floodplain.
In all but the steepest mountain cascades, rivers will meander. In lower gradient valleys, such as the Sacramento and San Joaquin for much of their length, a river’s physical imperative to meander is the greatest.
When rivers are denied the space to meander due to levees, rock revetments, or other impediments, many beneficial river services are diminished. In their natural state, rivers are dynamic ecosystems, supplying the floodplain with soil and nutrients for its diverse riparian habitats and in turn providing organic materials to aquatic species. The meandering river and its floodplain temporarily store excess floodwater and recharge ground water and reduce stream velocities.
Meander bends often form oxbow lakes that eventually fill with soil and vegetation and serve as traces of the river’s previous courses. These oxbows and the meandering river give definition to the land, providing a characteristic “riverscape.”
Beyond the ecological and aesthetic benefits, setback levees make sense economically as well. Allowing a river to meander avoids the expenses involved in maintaining the levees and rock revetments designed to hold a river in place. Further, when denied the space to meander, floodwaters are often “funneled” down stream, leading to increased flood damage, greater downstream shoreline erosion and channel incision.
Once a levee is setback, the river may begin to meander and this poses a challenge to implementing riparian restoration on the floodplain. River Partners works with engineers to plan the restoration design. For example, Eric Larsen of UC Davis has constructed model scenarios that simulate the future migration of the river given different restraint conditions. The application of such models is an integral part of the scientific approach that River Partners takes to restoring native vegetation on dynamic riparian floodplains.