Stream power influence on southern Californian riparian vegetation

Mechanical damage by floodwaters is frequently invoked to explain the distr ibution of riparian plant species, but data have been lacking to relate veg etation to specific estimates of flood damage potential. This research uses detailed estimates of unit stream power (an appropriate measure of the pot ential for mechanical damage) in conjunction with vegetation cover data to test this relationship at 37 valley-bottom sites in the Transverse Ranges o f Southern California. A computer program, HEC-2, was used to model the slo pe and the variation in flow depth and velocity of the 20-yr flood across t he sites. Regression models tested the influence of stream power (and of he ight above the water table) on the woody species composition of 393 4-m cro ss-section segments of the valley-bottom sites. Results indicate that unit stream power does have a significant effect on the riparian vegetation, but that the amount of that influence and its importance relative to the influ ence of height above the water table varies between watersheds. Some specie s are found primarily in locations of high stream power, while others are l imited to portions of the valley bottom that experience only low stream pow er.