Thermal Suitability of the Los Angeles River for Cold Water Resident and Migrating Fish Under Physical Restoration Alternatives

Reza Abdi,Kristine Taniguchi-Quan,Jordyn M Wolfand,Eric D Stein,Katie Irving,Ashley Rust,Terri S Hogue,Daniel Philippus

doi:10.3389/fenvs.2021.749085

Abstract

Anthropogenic development has adversely affected river habitat and species diversity in urban rivers, and existing habitats are jeopardized by future uncertainties in water resources management and climate. The Los Angeles River (LAR), for example, is a highly modified system that has been mostly channelized for flood control purposes, has altered hydrologic and hydraulic conditions, and is thermally altered (warmed), which severely limits the habitat suitability for cold water fish species. Efforts are currently underway to provide suitable environmental flows and improve channel hydraulic conditions, such as depth and velocity, for adult fish migration from the Pacific Ocean to upstream spawning areas. However, the thermal responses of restoration alternatives for resident and migrating cold water fish have not been fully investigated. Using a mechanistic model, we simulated the LAR’s water temperature under baseline conditions and future alternative restoration scenarios for migration of the native, anadromous steelhead trout in Southern California and the historically resident Santa Ana sucker. We considered three scenarios: 1) increasing roughness of the low-flow channel, 2) increasing the depth and width of the low-flow channel, and 3) allowing subsurface inflow to the river at a soft bottom reach in the LA downtown area. Our analysis indicates that the maximum weekly average temperature (MaxWAT) in the baseline condition was 28.9°C, suggesting that the current river temperatures would act as a limiting factor during the steelhead migration season and habitat for Santa Ana sucker. The MaxWAT dropped about 3%–28°C after applying all the considered scenarios at the study site, which is 3°C higher than the determined steelhead survival threshold. Our simulations suggest that without consideration of thermal restoration, restoring hydraulic conditions may be insufficient to support cold water fish migration or year-round resident native fish populations, particularly with potential river temperature increases due to climate change.

Highlights

Like many urban rivers, the Los Angeles River (LAR) is experiencing a renaissance and is viewed as a valuable ecosystem to be restored as a community amenity as opposed to the old paradigm where it was considered a source of flooding to be controlled and a conveyance for treated wastewater to the ocean (Beach, 2001; Everard and Moggridge, 2012)
The machine learning (ML)-based predictions of river temperature in the migration season for upstream and downstream of the study area showed that the calculated thermal metrics, Maximum Weekly Maximum Temperature (MaxWMT) and Maximum Weekly Average Temperature (MaxWAT), exceeded the recommended 20°C MaxWMT for fish corridors and critical maxima survival threshold of 25°C for the steelhead, which is above the 22°C maxima for the sucker (Figure 2)
One explanation could be that the upstream station is in a soft bottom portion of the river while the downstream station is after 20 km of bare concrete channel, which can increase water temperature (Sun et al, 2016)

Summary

Introduction

The Los Angeles River (LAR) is experiencing a renaissance and is viewed as a valuable ecosystem to be restored as a community amenity as opposed to the old paradigm where it was considered a source of flooding to be controlled and a conveyance for treated wastewater to the ocean (Beach, 2001; Everard and Moggridge, 2012). The overall goal is to improve ecological function through targeted remediation efforts to provide a more ecologically dynamic state (Bernhardt and Palmer, 2007). This aligns with similar projects across the world where there is an integrated and pragmatic approach to urban river restoration to improve biodiversity and achieve overall ecosystem function and resilience (Palmer et al, 2005; Smith et al, 2014; Chou, 2016). For the LAR, improving river connectivity for steelhead from the sea to their native spawning grounds in southern California is a priority ecological endpoint (City of Los Angeles, 2007). Stream temperature influences the distribution of fish, food availability, body growth, movement, fecundity, and spawning success (Caissie, 2006)

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Conclusion