Stream quality in the San Lorenzo River Basin, Santa Cruz County, California

Stream quality in the San Lorenzo River basin, Calif., was studied from November 1973 through June 1975 to determine areal and temporal variations in water quality, problem areas and times of water quality degradation, and compliance with State and local water-quality objectives. Sampling from November 1973 through July 1974 was done at five stations in the Zayante Creek drainage basin and at one station on the San Lorenzo River at Big Trees. Sampling from August 1974 through June 1975 was done at 15 stations in the San Lorenzo River basin, including one station in the Zayante Creek subbasin. Properties and constituents measured were water temperature, specific conductance, pH, alkalinity, dissolved oxygen, turbidity, fecal-coliform and fecal-streptococcal bacteria, major dissolved ions, nutrients (nitrogen and phosphorus), chemical and biochemical oxygen demand, and trace elements. Benthic invertebrates were collected during autumn 1974 and spring 1975 at two stations.

Results show that the water in most streams in the basin can be classified as calcium bicarbonate, but based on the dissolved-ion concentrations the basin can be divided into three water-quality areas that correspond generally to three geologic areas. The most pronounced change in water quality occurs during storms when streamflow, turbidity, and total Kjeldahl-nitrogen, total-phosphorus, dissolved-potassium, and fecal-coliform concentrations increase, while concentrations of most dissolved ions decrease owing to dilution of the nonstorm part of the flow. Late winter and early spring nonstorm flows yield water of the best quality because the concentrations of most dissolved ions and fecal-coliform and fecal-streptococcal bacteria are relatively low, whereas turbidity values are the same as, or not appreciably greater than, during the summer low-flow period.

Dissolved-ion concentrations exceeded objectives at stations on east-side streams north of the Zayante fault, probably because of lower-than-normal streamflows during water year 1975. Total-nitrogen and fecal-coliform concentrations exceeded objectives in the Zayante Creek and Branciforte Creek drainage basins, probably because of domestic sewage from improperly operating septic-tank systems. The high total-nitrogen and fecal-coliform concentrations in the Zayante Creek basin might also result from the primary-treated effluent discharged by the city of Scotts Valley into an old sand pit.

Results of diel studies (intensive sampling for a 24-hour period) made at two stations on the San Lorenzo River indicate that residential areas between the stations do not contribute sufficient quantities of oxygen-demanding wastes to the river during low streamflow to cause appreciable dissolved-oxygen depletion. Greater streamflows and residential development seem to be responsible for unstable streambed substrates and reduced diversity of benthic invertebrates during the spring at a station downstream of most of the residential areas in the San Lorenzo River valley.