2009 witnessed the debut of a multi-agency program to assess the health of streams in southern California. Although great effort was expended by earlier bioassessment programs of local agencies, they were project-specific and could not produce an assessment of the region as a whole. Through collaborations, leveraging of resources, and reallocations of existing efforts, the Stormwater Monitoring Coalition developed the first large-scale probabilistic assessment of over 7000 kilometers of streams in southern California's coastal watersheds. At 121 sites, benthic macroinvertebrates, diatoms, soft algae, riparian wetlands, water chemistry, physical habitat, and water column toxicity were sampled. Analysis of preliminary data shows that the majority of stream kilometers were in good condition, although impacts were severe in certain locations for many indicators. For example, riparian wetlands were most degraded in urban areas, and macroalgae growth was heaviest in agricultural streams. Unexpectedly, toxicity to Ceriodaphnia was pervasive, including in streams with undeveloped watersheds. The relationships between natural and stressor gradients on three benthic communities (i.e., macroinvertebrates, diatoms, and soft algae) are investigated using multimetric indices. Relative risks of potential stressors are also evaluated. This collaborative program demonstrates that regional assessments are possible with minimal additional cost.