Of those, 207 square miles drain directly into the mainstem of the river without first entering into major regulated tributary rivers¹ Such as Tujunga Wash, Arroyo Seco, the Rio Hondo, and others. (herein referred to as “Direct Subwatershed”). The LA Regional Water Quality Control Board Basin Plan (Basin Plan)² Los Angeles Regional Water Quality Control Board, Basin Plan for the Coastal Watersheds of Los Angeles and Ventura Counties. Accessed on November 30, 2018. established 24 designated beneficial uses of waterbodies in the watershed. Water quality objectives were subsequently established to ensure the protection of such beneficial uses. The presence of these beneficial uses throughout the length of the 51-miles of the LA River is shown in detail in Chapter 2 of Appendix Volume II: Technical Backup Document.

Approximately 62% of the watershed is developed with mixed land uses. Pollutants including bacteria, nutrients, oil and grease, trash, and trace metals, typically generated from land use activities, can be mobilized by dry and wet weather runoff and transported into the LA River, leading to degraded water quality and creating negative impacts on the aquatic ecosystem as well as human use of the waterway.

Many waterbodies in the watershed, including the LA River itself are classified as impaired waters by the Clean Water Act³State Water Resources Control Board, Impaired Water Bodies, Accessed on September 20, 2018. and require “treatment” to support their designated beneficial uses established in the Basin Plan.

In an effort to restore impaired water bodies in accordance with Section 303(d) of the Clean Water Act, the State Water Resources Control Board, and the Regional Water Quality Control Board established Total Daily Maximum Loads (TMDLs), a regulatory level that sets the maximum pollutant amounts allowed to be discharged into an impaired water body. The LA River is subject to five TMDLs that collectively regulate wet and dry weather discharges of 13 pollutants including ammonia, bacteria, cadmium, copper, nitrate, nitrite, lead, selenium trash, and zinc. TMDL targets are established based on pollutant source assessments as well as human health and ecosystem toxicity analyses. As a result, TMDL targets vary spatially and temporally throughout the river. Water quality modeling and priority mapping in the map above represents an integrated evaluation of dry and wet weather runoff quality compared to impaired receiving water bodies, their identified beneficial uses and impairments, and land use-based pollutant loading rates.⁴County of Los Angeles Department of Public Works, IRWMP Appendix F: GLAC IRWMP Water Quality Objectives and Targets. Accessed on September 20 2018.

Considerable resources from the public and private sectors have been dedicated to improving the water quality within impaired waterbodies in the watershed. Two Watershed Management Programs (WMPs) and one Enhanced Watershed Management Program (EWMP) were developed under the 2012 LA County Municipal Separate Storm Sewer Systems Permit (MS4 Permit) to facilitate watershed-wide implementation and strategies for TMDL compliance. EWMP/WMP progress metrics were established based on storage required to achieve pollutant load reduction targets. Capacity achieved within the direct subwatersheds was aggregated to create the EWMP/WMP target ruler to show that planned and/or completed projects helped to nearly meet the requirements set forth in the 2012 MS4 permit, although there is remaining uncertainty in the funding and implementation of these plans to keep pace with the approved planned milestone.

As a participating agency of all three EWMP/ WMPs in the LA River watershed, LA County continues to work with other EWMP/WMP agencies to identify and develop water quality improvement projects or programs along the LA River to capture stormwater and urban runoff for treatment, infiltration, or direct use, and support the designated beneficial uses of the LA River.

Revised EWMP/WMPs have been prepared in 2020 and 2021 and are still under draft review by the Los Angeles Regional Water Quality Control Board (LARWCQB) as of December 2021. Once finalized later in 2022, it is expected that updated numeric targets, watershed modeling methods, and accounting for projects developed over nearly a decade since the original publications will be provided. These revised plans were prepared alongside LA County’s Safe, Clean Water Program (Measure W), which has streamlined funding for EWMP/WMP projects in the LA River Watershed and throughout LA County.