San Gabriel River Regional Monitoring Program

Biotic Condition

The overall condition of streams in the San Gabriel River Watershed was assessed by collecting benthic macroinvertebrates (BMIs) during dry weather conditions in the spring and summer from 84 sites over a nine year period from 2006 to 2014. BMI communities provide an integrated view of stream water quality over time since their juvenile forms spend most of their lives living on the stream bottom or water column. The biotic condition for each site was determined using the California Stream Condition Index (CSCI).


  1. Mazor, R.D., A. Rehn, P. R. Ode, M. Engeln, K. Schiff, E. Stein, D. Gillett, D. Herbst, C.P. Hawkins. 2016. Bioassessment in complex environments: designing an index for consistent meaning in different settings. Freshwater Science, before publication.
  2. Rehn, A.C.,R.D. Mazor, P.R. Ode, 2015. The California Stream Condition Indices (CSCI): A New Statewide Biological Scoring Tool for Assessing the Health of Freshwater Streams. SWAMP Technical Memorandum. SWAMP-TM-2015-0002.
The California Stream Condition Index (CSCI) is a new statewide biological scoring tool that translates complex data about benthic macroinvertebrates (BMIs) found living in a stream into an overall measure of stream health. The CSCI combines two separate types of indices, each of which provides unique information about the biological condition at a stream: a multi-metric index (MMI) that measures ecological structure and function, and an observed-to-expected (O/E) index that measures taxonomic completeness. Unlike previous MMI or O/E indices that were applicable only on a regional basis or under-represented large portions of the state, the CSCI was built with a statewide dataset (n = 1,985 sites) that represents the broad range of environmental conditions across California.

On the map to the left you can explore the biological condition of each region of the San Gabriel watershed by viewing pie charts that depict the proportion of stream reaches that were found to be in one of the following condition categories:


  1. ≤ 0.62 Very Likely Altered
  2. 0.63 to 0.79 Possibly Altered
  3. 0.80 to 0.92 Possibly Intact
  4. 0.93 to ≥1.0 Likely Intact
The CSCI was calibrated during its development so that the mean score of reference sites is 1. Scores can range from 0, indicating altered biological condition to > 1. In practice, CSCI scores observed from nearly 2000 study reaches sampled across California range from about 0.1 to 1.4. The four categories presented above were based on the 30th; 10th; and 1st percentiles of CSCI scores at reference sites. While these ranges do not represent regulatory thresholds, they provide a useful method for interpreting CSCI results.