Flooding and Riparian Vegetation and Nutrient Cycling in the Grand Canyon

Katy Dynarski

Abstract: Terrestrial ecosystems are complicated, dynamic systems affected by physical, biological, and chemical processes. Flooding of riparian ecosystems can affect all three of these areas: floods redistribute sediment to alter physical habitats, scour away or bury vegetation, and stimulate nutrient mineralization via the burial organic matter. The Grand Canyon of the Colorado River experienced annual spring floods prior to the construction of the Glen Canyon Dam. Because the dam controlled river flows and ended yearly flooding, it quickly changed the nature of the river’s adjacent terrestrial ecosystems. Novel riparian ecosystems established at lower elevations in the canyon because they were no longer scoured away by yearly floods. Controlled flooding (or high flow experiments) in the canyon has been used to attempt to manage riparian vegetation; goals for vegetation management have included returning the riparian zones to their pre-dam state and to remove riparian vegetation from sandbars that could otherwise be used for camping. These floods were mostly unsuccessful at achieving these vegetation management goals, with no loss of riparian zone vegetation lasting for more than a year after each high flow experiment. Flooding had a positive effect on nutrient availability in both groundwater and surface waters in the river, suggesting that periodic flooding may be important for the productivity of both terrestrial and aquatic ecosystems in the Grand Canyon. Further, if riparian ecosystems are considered as something to be valued rather than eradicated, periodic flooding of the Colorado River seems to be an important management tool.