Assessing Biological Impacts from Storm Flow Diversions: A Case Study

Herein are described methods and results of analysis of impacts on biological resources in the Upper Arroyo Seco that may result from storm flow diversions associated with the implementation of the proposed Pasadena Water and Power (PWP) Arroyo Seco Canyon Project (ASCP). The ASCP would repair, replace, and/or rehabilitate the diversion and intake structures in the Arroyo to maximize the capture of PWP’s surface water rights. Two-dimensional hydraulic models were prepared of the Devil’s Gate Reservoir area for representative wet, average, and dry years for the existing and with-diversion scenarios. Hydraulic modeling indicates that only during dry years do with-diversion conditions have expected changes to maximum reservoir pool depth. Within the stream section of the study area modeling showed little or no measurable change in hydraulic conditions (i.e. depth, velocity, inundation area, and shear) with project diversions. Additional sediment transport modeling and groundwater analysis indicated no impacts from proposed conditions. An ecological modeling study was prepared for the study area. The ecological effects of flow diversions were assessed based on modeled relationships between proposed hydraulic changes and the probability that the resultant physical conditions can support southern willow riparian habitat conditions conducive to occupancy by least Bell’s vireo. A statistical approach, rather than a mechanistic approach, was used to estimate the probability of occurrence of vireo under physical conditions associated with flow diversion scenarios. All hydraulic variables tested showed significant statistical differences (p<0.001) in willow occurrence between predicted future conditions and historic flow characteristics. Given the small sample size of vegetation polygons and the high number of hydraulic observations, it is believed the significance estimates may be overestimated. The models predict a slight decrease in probability of occurrence of willow habitat in dry years in the proposed condition. However, net changes in probability are less than 10%.