The LA River carries water and discharges it to the Pacific Ocean 365 days a year. LA River dry weather flow is a consistent supply source, however, there is uncertainty in its future. In 2020, more than 50,000 acre-feet per year (AFY) of dry weather flow is discharged through the river and into the ocean in Long Beach. Although planning and analyses are underway, it is conservatively estimated that up to 10,000 AFY of dry weather flow can be diverted at one or multiple site locations along the LA River and conveyed for treatment for groundwater recharge or local reuse. Limited beneficial uses have been identified in the river downstream of Downtown LA (RM 24) and, therefore, some level of diversions downstream of river mile 24 potentially cause little to no harm to areas downstream. As well, diversion of dry weather flows prior to the estuary portion would have a beneficial impact on estuary resources as the continuous input of fresh water has diluted the brackish water upon which estuarine species rely for various life stages.

Wet weather flow is a certain but highly inconsistent supply source. Annual wet weather volumes range from 50,000 AF in the driest years to nearly 1,000,000 AF in the wettest years.

The temporal variability in rainfall poses some challenges. However, technical solutions exist that can create an efficient process for capturing, diverting, storing, treating, and ultimately recharging our local groundwater aquifers with stormwater. Where possible in the watersheds and along the tributaries, water should be captured and allowed to percolate into the groundwater table. However, once flows reach the mainstem of the LA River, the opportunities for infiltration are mostly lost unless water can be safely diverted from the channel for potential recharge or reuse. Operating a water treatment facility to treat fluctuating flows from sporadic rainfall events can be technically challenging. The figure Wet Weather Capture Rates with Active Storage depicts various scenarios of diversion rates from the LA River with and without temporary storage. It has been estimated that without storage, yields upwards of 30,000 AFY of treated water are available for groundwater recharge. These yields increase significantly with the implementation of centralized and distributed storage projects upstream along the river and in the watershed. Over time, if approximately 5,000 AF of active storage can be developed through upstream surface projects (reoperation of Sepulveda Basin, implementation of river parks, tunnels, detention basins, etc.), estimated capture volumes could yield nearly 50,000 AFY of treated water.

In the San Fernando Basin, the City of LA Department of Water and Power, LA County Flood Control District, and LA County Public Works are cooperating to increase capture of stormwater for groundwater recharge through projects such as the Tujunga Spreading Grounds Enhancement Project. In the Central and West Coast Basins, continued pressure on imported water may increase pumping in the basins, leading to new needs for recharge. At the Los Angeles and Montebello Forebays, permeable soils are exposed at ground surface allowing groundwater to percolate into the aquifer. Additionally, many areas of the basin may be suitable for recharge via injection wells.

A regional water recharge system concept could include upstream storage using infrastructure and park spaces, a diversion facility downstream of Downtown LA, treatment facilities, a groundwater injection wellfield, and a discharge facility to manage brine from the treatment process. It is envisioned that the brine could be sent directly to the sanitary sewer, yet studies show that there is potential for the brine to be released back into the river to improve habitat conditions in the estuarine reaches of the river below river mile 9.