Dry weather inflows are comprised of incidental urban runoff entering the river through storm drain outfalls, flows from the three water reclamation plants (WRP), and groundwater upwelling that occurs in the soft bottom reaches in the Glendale Narrows. In most reaches of the river, dry weather flows are primarily contained within the low flow channel, a shallow channel typically in the center of the river in concrete sections.
Although dry weather flows provide water for invasive vegetation species to grow with limited control, they also help create recreational and ecological opportunities within the LA River. In recent years, agencies and municipalities have expressed intent to increase reuse of water treated at WRPs thereby decreasing discharges into the river, to improve groundwater management, and to reduce urban runoff. Combined, these actions could change the current beneficial uses of the existing dry weather flows within the river due to reductions in the total amount of flow. In response to this challenge, the State Water Resources Control Board along with the LA Regional Water Quality Control Board (together, “the Water Boards”), both of which support maximizing the use of recycled water and protecting beneficial uses, embarked on a multi-year study in 2019, to balance the impact of reuse and instream needs. This study may be completed in 2021 or 2022 when in-stream dry weather flow management will be revisited.
Upon review of the dry weather inputs and their potential for reductions in the future, it is conceivable that future dry weather flows could approach zero. A possible future dry weather flow scenario was estimated by assuming that all three WRPs recycle 100% of their effluent, and groundwater upwelling and urban dry weather runoff are significantly reduced, resulting in a possible future dry weather flow of just a trickle at the mouth of the river.
Existing, Possible, and Plausible Wet-Weather Flows
However, understanding the Water Boards’ process for balancing the future low flow regime in the LA River, a plausible flowrate for future dry weather flows could be about 15 cubic feet per second – an 80% reduction in the estimated existing dry weather flow today.
Based on the plausible future dry weather flow, adjustments to the dimensions or shape of the low flow channel may be needed to maintain recreation or habitat conditions in certain reaches of the river.
In areas where a wider flow is desirable for habitat (such as the lower river algae mat areas), the low flow channel can be partially filled in.
In areas where a deeper flow is desirable for recreation (such as kayaking) a narrower low flow channel or other innovative ideas can provide this beneficial use with less water.
It may also be advantageous to have significantly less dry weather flow in soft bottom reaches in summer months to reduce the presence of invasive species that thrive on a continuous water flow.