SEDIMENT REGIME CONSTRAINTS ON RIVER RESTORATION – AN EXAMPLE FROM THE LOWER MISSOURI RIVER

Dammed rivers are subject to changes in flow regime, water-quality regime, sediment regime, and channel morphology. Each of these stresses may contribute to diminished aquatic habitat quality and quantity, and each presents restoration opportunities. Of the four, the sediment regime remains a particularly unyielding challenge on many dammed rivers. The magnitude of the challenge is illustrated on the Lower Missouri River (LMOR) where the largest water storage system in North America has decreased the downriver sediment load to 0 to 24 percent of pre-dam values. In response, the LMOR channel has degraded in places and aggraded in others. Degradation and aggradation limit what can be accomplished by flow-regime management: in degrading river segments, ecologically desirable reconnection of the flood plain has become an almost impossible goal, whereas in aggrading river segments, even small spring pulses threaten to saturate farm lands. Lack of sediment in degrading river segments also limits sustainable restoration of sand-bar habitat for listed bird species. In stable to aggrading segments, there is concern that ongoing bed adjustments may alter the designed hydroperiod in fish-habitat restoration projects. Effective restoration strategies on dammed rivers require integrated, predictive understanding of ongoing geomorphic adjustments to altered sediment and flow regimes, and how these adjustments relate to restoration objectives. Because sediment-management approaches (for example, gravel replenishment or reservoir bypass systems) are very costly, restoration goals may need to accommodate an alternative future with a permanently altered sediment regime.