The Los Angeles River Environmental Flows Project: Balancing Water Reuse and Ecological Support Goals in an Effluent Dominated River

Flows in urban rivers should be managed to both support water supply needs and protect and/or restore instream ecological functions, goals that are often in opposition to each other. In this study, we worked with the State Water Resources Control Board and the Los Angeles Regional Water Quality Control Board, and in cooperation with local municipalities, to assess the impacts of water management decisions on environmental flows in the Los Angeles River. We investigate: (1) how do changes in wastewater reuse and stormwater management impact river flows and subsequently the viability of ecosystems and beneficial uses in the river? (2) how can in-stream channel restoration mitigate these impacts? and (3) what tools and processes can be developed to provide guidance for developing recommended flow criteria in other urban rivers? We developed hydrologic, hydraulic, ecology, temperature, and water quality models of the basin to investigate how changes in wastewater reuse, stormwater management, and in-channel habitat restoration impact river flows and subsequently the viability of ecosystems and beneficial uses in the river. We used a functional flow approach, piloting the California Environmental Flows Framework (CEFF) and demonstrating that CEFF can be applied in a highly urbanized watershed where flow alteration is primarily caused by treated wastewater and stormwater discharges. Sensitivity curves that relate potential changes in treated wastewater discharge to changes in functional flows show that a 4% decrease in current treated wastewater discharge may negatively impact habitat for indicator species during the dry season. More opportunity exists for wastewater reuse during the wet season, when current treated wastewater discharge may be reduced by 24% with minimal impacts to ecology and recreation. Initial results show that in-stream restoration (e.g., low flow channel modification, changes to channel substrate) has the potential to offset some of the negative impacts of increased wastewater reuse for indicator species during the dry and wet seasons. The developed approach has potential to inform similar tradeoff decisions in other urban rivers where flows are dominated by wastewater or storm drain discharge.