Water-Quality Monitoring in the Sepulveda Flood Control Basin of the Los Angeles River Watershed Active

The Los Angeles River

The Los Angeles River flows from its headwaters in the Angeles National Forest to the Pacific Ocean in Long Beach, California. The river is 51 miles long and winds through 14 cities. The Los Angeles River is managed for multiple uses. In one section, a concrete runoff ditch helps control flooding. The natural river is a source of recreation for the 4.4 million people living in the surrounding area. Revitalization is planned for the entire reach of the Los Angeles River that runs from river mile 51.0 in Canoga Park within the City of Los Angeles to mile 0 at Long Beach where the river meets the Pacific Ocean. Critical riparian and freshwater marsh habitat will be restored, as will more natural hydrologic processes.

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To support revitalization and restoration efforts, the U.S. Geological Survey (USGS) is providing continuous water-quality data within the Sepulveda Flood Control Basin. Data are expected to provide information for enhanced recreational use, inform local organizations and municipal entities interested in current water-quality and streamflow conditions, contribute to future assessments of factors affecting surface-water quality in this heavily urbanized area, and support science-based management practices to protect public land and water. The data from this study could provide a correlation point for discrete sampling locations in the Sepulveda Flood Control Basin and create a baseline for future hydrologic and ecosystem investigations.

Sepulveda Flood Control Basin Sediment Study

A sediment study was added during the period of July 1, 2021, as part of the cooperative water-resources program between the Mountains Recreation and Conservation Authority (MRCA) and the U.S. Geological Survey (USGS). The sediment study will continue through September 30, 2024. The MRCA wants to further explore sediment-management strategies in the Sepulveda Flood Control Basin; the sediment and surface-water data collected for this study (table 1) will provide information to better understand the physical system of the Los Angeles River in the Sepulveda Flood Control Basin. These data will be published in the USGS National Water Information System (NWIS) and can be used in future studies involving streamflow and transport models.

The study area (fig. 1) is in the Sepulveda Flood Control Basin from the headwaters of the Los Angeles River to USGS station 11092450 which monitors flow from the Sepulveda Dam. Baseflow sediment sampling at Los Angeles River at Sepulveda Dam, California (USGS station 11092450), was added in water year 2021 to help understand sediment transport through Sepulveda Dam, the downstream end of the Sepulveda Flood Control Basin. Stream stage data are being collected using radar on the Los Angeles River at Burbank Boulevard near Van Nuys, California (USGS station 341015118284001), and stream stage data are being collected using a pressure transducer on Bull Creek near Van Nuys, California (USGS station 341049118295201). Some tributaries only flow after rain creates runoff, so continuous water-quality monitoring data are being collected during the rainy season (November through April). New stations were established in water year 2022 (table 1) to monitor sediment transport at other points along the Los Angeles River and its tributaries. Sediment transport data are needed for tributaries to identify areas with high-sediment yield versus those with lower-sediment yields and to support sediment-management strategies for Sepulveda Flood Control Basin.

Sediment Study Objectives

The lack of streamflow and sediment-transport data within the Sepulveda Flood Control Basin has complicated efforts to develop and identify environmental benefits such as sediment sequestration, alleviation of scour, or potential for aquatic organism habitat in proposed restoration design efforts.

Existing and planned USGS sediment-monitoring stations will provide data that local stakeholders can use to (1) improve the understanding of streamflow, stream stage, and sediment transport rates; (2) develop models; and (3) design restorations for the Los Angeles River and its tributaries within the Sepulveda Flood Control Basin.

Adding sediment sampling downstream at Los Angeles River at Sepulveda Dam, California (USGS station 11092450), will help quantify sediment transport through the Sepulveda Dam. Additional sediment-transport data collection at tributaries can be used to identify areas with high and low sediment yields. The sediment data collected will be available from the USGS National Water Information System (NWIS).

Primary data gaps were identified in the Feasibility Study for Restoration of the Los Angeles River in Sepulveda Basin (https://www.theriverproject.org/sepulveda-basin-es):

• the distribution of storm runoff
• sediment transport into the Sepulveda Flood Control Basin from the main-stem Los Angeles River and each tributary.

This study is the first step in addressing these data gaps, but more data are needed to inform long-term management decisions on this part of the Los Angeles River.

USGS Sediment and Water-Quality Monitoring

In water year 2022, sediment, specific conductance, pH, dissolved oxygen, water temperature, and turbidity data were collected at Los Angeles River at Sepulveda Dam, California (USGS station 11092450; table 1).

In water year 2023, sediment data were collected along with specific conductance, pH, dissolved oxygen, temperature, and turbidity data year-round at Los Angeles River at Sepulveda Dam, California (USGS station 11092450). Also, sediment and several continuous water-quality parameters (specific conductance, pH, temperature, and turbidity) are being collected at other stations on the Los Angeles River (USGS station 341015118284001) and on Bull Creek (USGS station 341049118295201). Data will be processed, reviewed, and approved in a timely manner using standard USGS methods.

In water year 2024, data collection will continue at Los Angeles River at Sepulveda Dam, California (USGS station 11092450), and at new stations that will be established in water year 2024. Collection of sediment, stream stage, specific conductance, pH, temperature, and turbidity data will continue at these stations during storm events.

Los Angeles River Monitoring Stations

Los Angeles River at Sepulveda Dam, California (USGS station 11092450) is a continuous water-quality monitoring station along the Los Angeles River. This project is part of the Urban Waters Federal Partnership.

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