Characterizing relationships between dissolved oxygen and riffle crest depths in northern California streams

During California's dry season, the natural recession of streamflow often coincides with declines in dissolved oxygen (DO) concentrations, impacting sensitive aquatic species such as salmon. Thus, understanding the relationship between DO and streamflow is important to evaluate risks and set water quality standards. We explored the relationship between minimum DO and riffle crest thalweg (RCT) depth, a proxy for streamflow. We analyzed daily data collected between May to October in 26 pools within streams in a northern California coastal watershed over a six-year period. We first developed statistical models to characterize the local relationship between DO and RCT depth at each pool and then developed a regional model using all observations to predict when DO falls below a critical threshold for salmonids (6.5 mg/L). Observations indicated a mean (range) daily minimum DO value of 7.33 (0.01-10.95) mg/L with a mean RCT of 9.07 (0.10 to 34.63) cm across all sites and pools. On average, the first date that minimum DO fell below the critical threshold was July 8th (May 13th to August 31st). The proportion of DO levels below the 6.5 mg/L threshold increased sharply once RCT fell below about 7 cm, and more than half of all DO measurements were below this threshold when RCT depth was between 3 and 6 cm. Our local model highlighted a significant, non-linear and positive relationship between DO and RCT depth at all sites (mean R² = of 0.72). The regional model correctly predicted DO values above and below the threshold 78 % of the time, with a mean predicted first date below the threshold of August 5th at a mean RCT of 4.25 cm. In addition to RCT depth, we found that the day of the year, geology, and stream temperature were all important factors for explaining DO variability across watersheds and overtime. Our results can aid managers to predict the risk of DO falling below critical biological thresholds and maintain streamflow to support salmon and other sensitive aquatic species.