Ecohydrologic Effects of Stream Restoration
Stream restoration efforts, particularly within meadow systems, increasingly rely on “pond and plug” type methods in which (a) alluvial materials are excavated from the floodplain, forming ponds; (b) excavated alluvial materials are used to plug incised channels; and (c) smaller dimension channels are restored to the floodplain surface. Despite the large number of “pond and plug” restoration projects undertaken in the western United States, little research has been conducted to evaluate and quantify the effects of such topographic modification upon hydrology and riparian vegetation in these systems. To predict the changes in hydrologic processes and the distribution of commonly found meadow riparian plant species a hydrologic model and a suite of individual vegetation species models were used in concert. First we developed, calibrated and validated a hydrologic model of a 230 ha mountain meadow along a 3.6 km restored reach of Bear Creek in northeastern California, and used it to simulate the pre- and post-restoration topographic conditions. Next, vegetation data from 170 plot locations distributed throughout the study area were combined with simulated water table depth time series to develop species distribution models for individual plant species. In each vegetation model the probability of occurrence predicted as a function of growing season water table depth and range. Last, hydrologic and vegetation models were jointly used to predict the spatial distribution of individual plant species for pre- and post-restoration conditions. Our results document three general hydrologic responses to the meadow restoration effort: 1) increased groundwater levels and volume of subsurface storage; 2) increased frequency/duration of floodplain inundation and decreased magnitude of flood peaks; and 3) decreased annual runoff and duration of baseflow. Vegetation modeling results indicate an increase in the spatial distribution of obligate wetland, and facultative wetland plant species, as well as a decrease in the distribution of facultative upland and obligate upland plant species. This study supports and quantifies the hypothesis that “pond and plug” type stream restoration projects have the capacity to re-establish hydrologic processes necessary to sustain riparian systems. The methods utilized could be used to improve realistic objective setting in similar projects in similar environments, in addition to providing a quantitative, science-based approach to guide riparian restoration and active re-vegetation efforts.