Riparian systems are controlled by interacting hydrologic and geomorphic processes. Floods may alter river channel characteristics and the extent of riparian vegetation while enhancing recruitment of riparian species and recharging the alluvial water table. Geomorphic features, such as canyons and valleys, control the size of the riparian zone, as well as depth of the water table. Driving variables may differ from north to south, especially hydrology. For example, northern riparian zones are influenced by ice scour, while southern zones often have flash floods.
Riparian systems occur along spatial and temporal gradients.
Along elevational gradients, riparian vegetation may change from simple deciduous forests to mixed deciduous to coniferous and possibly alpine wetlands. Differences among channel, terrace, and upland plant communities decrease with increasing elevation as moisture stress decreases. Western riparian ecosystems have been greatly altered by human activity. Better understanding of the ecology of western riparian ecosystems will increase potential for restoration and protection of remaining areas.