San Gabriel River Regional Monitoring Program

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What were swimming conditions during the summer of 2015?

During the summer of 2015 a total of 160 water samples were collected from the upper watershed swimming sites and analyzed for E. coli. Of these, 15% (n = 24) exceeded the single-sample recreational water standard for E. coli (235 MPN/100 mL). This number of exceedances was second only to the summer of 2014 when 17% exceeded standards.
The majority of the bacterial exceedances in 2015 were again measured in the vicinity of the confluence of the West and North Forks of the San Gabriel River where recreational use is heavy, especially on the weekends and holidays. The greatest frequency of exceedances of the single sample standard occurred at the lower North Fork of the San Gabriel River (SGUT201) where 56% of the samples (n = 10) exceeded the standard. Station SGUT210, located downstream of SGUT201 on the West Fork had the next greatest number of exceedances (n = 6; 33%). This site was added in mid-June 2014 after bioassessment crews found it littered with trash and diapers, and with graffiti on many of the large boulders.

The concentrations of bacteria were much lower on the East Fork of the San Gabriel River and at Cattle Canyon, a tributary to the East Fork. These stations are also used heavily by the public for recreation. The reasons for the lower bacteria counts here are not clear. Stations SGUT206 and SGUT207 on the East Fork at Cattle Canyon and Graveyard Canyon, respectively, each had two samples that exceeded the standard. Stations SGUT204 (upper E. Fork) and SGUT205 (Upper Cattle Canyon) had one exceedance each.

Levels of bacteria measured across all sites were greatest on the weekends and holidays. For example, the weekend days of July 18th and 26th had four exceedances and the Independence Day holiday (July 4th and 5th) had 3 exceedances each.