Episodic Channels: Effects of Regulation on Non-Equilibrium River Systems in California (Invited)

Mediterranean-climate rivers are characterized by episodic channels, whose geomorphic work is concentrated in short, infrequent events (large floods), separated by long periods of quiescence in which the channel narrows and riparian vegetation can establish and mature, only to be disrupted by the next large disturbance. While not ‘pretty’ in conventional terms, such rivers support diverse assemblages of native species, adapted to the episodic regime. Because of the importance of irrigated agriculture in Mediterranean-climate regions, large reservoir storage projects are common, resulting in dam-induced reductions flood peaks, which have reduced dynamism in downstream channels. The result has been loss of habitat diversity and native species. A systems-level analysis of the Sacramento-San Joaquin and other rivers reveals that Q2 has commonly been reduced by 80%, sediment loads reduced, and vegetation encroached in formerly active channels. More profound have been hardening of banks and isolation of floodplains by levees. Restoration of ecological values in such rivers will require room for the river to move and flood, as well as floods sufficient to drive these processes. We identify a set of rivers with highest potential for re-activation or preservation of dynamic process in California.