Riparian vegetation responses: Snatching defeat from the jaws of victory and vice versa

The 1996 controlled flood failed to demonstrate five aspects of its primary vegetation-related management goal of removal of near-shore vegetation. First, when compared to pre-flood measurements, total vegetative cover was reduced only 20% and the areal extent of wetland and woodland/shrubland patches was not significantly different from the previous year when measured 6 months after the high flows. There was an immediate effect in terms of burial of some marshy areas under coarse sand, but most of these recovered within 6 months. Second, the controlled flood consistently affected only the lowest vegetation layer (grasses and herbs). Third, there was some effect on soil seed banks; sites lost roughly 45% of the seeds and 30% of the species richness of the pool of readily germinable seeds in the top 10 cm of the soil. Fourth, the loss of surface organic matter (duff) was significant in only 3 of the 9 sites, the other 6 showed no significant differences between years. There was no significant change across all sites. Finally, although there was no consistent effect on germination site quality in terms of mean soil grain sizes, there was a significant homogenization of substrates within and among sites due mostly to the loss or burial of fine sediments in return current channel settings. As documented in other chapters of this volume, the controlled flood was a success administratively and a successful demonstration of other management goals, especially in moving sediment from the channel bottom to high elevation deposits. Further, the flood was also a success in that it provided a relatively consequence-free opportunity to learn about flood hydrographs and vegetation in this system. Our data and those of others in this volume suggest that had the flows been successful in removing plants and reworking the underlying substrate in wetland patches, the recovery of vegetation would have been slowed considerably by the lack of fine, nutrient-rich sediments.