Like many urban rivers, the Los Angeles River (LAR) is experiencing a renaissance and is now 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). Given that the LAR flows through one of the largest and most urbanized cities in the United States, complete restoration to an undisturbed condition is not achievable through reclamation efforts. Instead, 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).

Identifying an ecological endpoint is one of the standards for successful river restoration (Palmer et al., 2005; Zhang et al., 2018; O’Brien et al., 2020). The anadromous steelhead trout (Oncorhynchus mykiss) is classified as endangered in southern California (U.S. Department of Commerce, 1997). 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). Connecting urban rivers to healthy reaches can be successful as flow and sediment are more likely to be in balance and the fish can exploit new areas (Findley and Taylor, 2006). In addition to meeting the trout’s physical habitat requirements, because fish are ectotherms, water temperature must be within a defined thermal range for migrating fish to survive. Stream temperature influences the distribution of fish, food availability, body growth, movement, fecundity, and spawning success (Caissie, 2006).

In this study, we assessed how physical restoration scenarios focused on improving connectivity will alter stream temperatures to better support migrating steelhead. To do so, we evaluated how restoration measures within the LAR may improve stream temperature during steelhead migration and support other native fish habitat from January through June which is primarily the migration season in southern California (Moyle et al., 2008). Stream temperature is a function of flow, depth, velocity, and substrate connections, all of which may be altered during the LAR stream restoration. Gu and Li (2002) found that the sensitivity of stream temperature to river flow rate is as significant as that to weather. Therefore, the other physical parameters that are controlled by flow (i.e., depth and velocity) also affect water temperature. Furthermore, water temperature in riverine systems within highly urbanized areas can be elevated through modifications in riparian landcover (by affecting the shading on the water surface), as well as surface and subsurface inflows (LeBlanc et al., 1997; Chen et al., 1998; Van Buren et al., 2000; Sridhar et al., 2004; Herb et al., 2009; Sun et al., 2015; Abdi and Endreny, 2019). Like other similar projects, remediation of the LAR has the best chance of success if efforts improve ecological function and the river can support self-sustaining populations (Palmer et al., 2005).

Fixing hydrologic and hydraulic conditions of urban rivers through physical modification is often the focus of river restoration projects (Barber and Gleason, 2017), yet seldom do restoration efforts look at how stream temperature can be improved for native fish. River temperature is a critical factor in riverine networks, as it controls the saturation of dissolved oxygen (Sand-Jensen and Pedersen, 2005; Null et al., 2017). In addition, while many rehabilitation projects do concentrate on improving water quality to address water contamination, stream temperature is often overlooked (Purcell et al., 2002; Walsh et al., 2005; Pander and Geist 2013). Rivers are often highly thermally polluted due to industrial discharges (e.g., thermoelectric power plants return flows; Madden et al., 2013) or due to associated land-use change e.g., deforestation and urbanization; (Parker and Krenkel, 1969; Wunderlich, 1972; Walsh et al., 2005; Poshtiri and Pal, 2016; Rogers et al., 2021). Increasing air temperatures from climate change are expected to increase river temperatures as well (Eaton and Scheller, 1996).

In addition to air temperature, substrate inflow as groundwater and hyporheic exchange regulates river water temperature during wet and dry weather (Risley et al., 2010; Kurylyk et al., 2016), which depending on site conditions (e.g., hard or soft bottom, and urbanized or forested area) and seasonality may vary (Poole and Berman, 2001). The upwelling in the LAR is important due to the condition of the river however, the hyporheic exchange inflow is negligible due to hardening of the floodplain (Paulinski et al., 2021). Further, riparian zone shade effects from tree canopy, hillslope, and buildings are also factors reducing river water temperature by providing terrestrial-based reduction in direct and diffuse solar radiation and the view-to-sky factor for the river, which influences longwave radiation (Boyd and Kasper, 2003). Recent advances in temperature modeling now provide the tools to explore how managing flows and riparian shading can influence thermal conditions within desired migration corridors.

This study aims to assess the river temperature condition for steelhead migration on the LAR mainstem and evaluate the cooling or warming effects of potential restoration scenarios. The applied restoration scenarios are suggested by the United States Bureau of Reclamation (U.S. Bureau of Reclamation, 2019): 1) increasing roughness of the low-flow channel (see Supplementary Figure S1 ¹ for an example of a low-flow channel) to reduce velocity, 2) increasing the depth and width of the low-flow channel in addition to increasing the roughness, and 3) applying additional subsurface upwelling to Scenario 2. Our central research questions are: 1) to what degree does river temperature limit steelhead migration in LAR mainstem from the Pacific Ocean to the soft bottom section of the LAR (Glendale Narrows)? 2) how does river temperature respond to restoration scenarios to facilitate steelhead migration? and 3) how would simulated thermal changes limit the year-round resident native fish in alternative restoration scenarios? Modeling results could provide a better understanding of the role of water temperature as a limiting factor for steelhead migration in the LAR. Our study had two hypotheses: 1) that warm water temperature is a limiting factor for cold water migrating fish in LAR and 2) the proposed USBR restoration actions cannot address the temperature problems for both species. Findings from this work will provide the LAR water managers with a more holistic understanding of the capabilities and consequences of river restoration alternatives.

We estimated the optimum thermal suitability ranges in the determined river reach of the LAR based on the desirable thermal condition for steelhead (see section 2.3). We then simulated the water temperature for current conditions and under the restoration alternatives for the LAR based on the considered thermal metrics.

Results from baseline condition simulations, and under potential restoration scenarios, showed that during migration season, baseline thermal conditions would not support the steelhead or resident Santa Ana sucker. Even with restoration of hydraulic conditions, temperatures would exceed their optimal thermal maxima. Water temperature should therefore be considered a limiting factor in facilitating steelhead migration on the LAR or establishing Santa Ana sucker populations. On average, water temperature was about 4°C higher than the fish’s threshold (25°C). Even though management scenarios could improve physical conditions, other plans should be considered to reach the desired temperature thresholds.

The ML-based predictions of river temperature in the migration season for upstream and downstream of the study area showed that the calculated thermal metrics, MaxWMT and MaxWAT, exceeded the recommended 20°C MaxWMT for fish corridors and critical maxima survival threshold of 25°C for the steelhead, which is also above the 22°C maxima for the sucker (Figure 2). The median of MaxWMTs was 28.9°C at station #4A and reached 33.6°C at station #4D, an increase of 16% over baseline condition. The median of MaxWATs was 27.2°C at station #4A and reached 28.2°C at station #4D, an increase of 4%. We observed a 30% decrease in the median of the calculated MinWMTs from 20.1°C to 13.7°C. The 16% increase and a 30% decrease in the median of MaxWMTs and MinWMTs metrics respectively, showed that the diel variations of the water temperature at the downstream station were broader compared to at the upstream station. 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).

By increasing the Manning’s roughness in the low-flow channel from 0.017 to 0.035 (Scenario 1), the average cross-sectional water column depth increased by 50% to 39 cm and the average flow velocity decreased by 55% to 0.44 m/s (see Figure 3; Supplementary Table S4 for more details). Under this scenario, the water profile in the river channel was elevated due to the increased Manning’s roughness coefficient, and the estimated average diel change in the river temperature, based on the defined thermal metrics (MaxWMT-MinWMT), decreased by 30% from 20.6°C to 14.5°C. The average MaxWMT in the simulated migration season for Scenario 1 decreased by 2.3°C–31.1°C (7% compared to the baseline condition) and the average MinWMT increased by 3.8°C–16.6°C (29% compared to the baseline condition; Figure 4). The average MaxWAT didn’t change significantly compared to the other two metrics and decreased by only 0.1°C–28.8°C (0.3% compared to the baseline condition) and it was 3.8°C higher than the determined steelhead survival line (Table 2). As reported by Smith and Lavis (1975) and Ahmadi-Nedushan et al. (2007), the decrease and increase in the average MaxWMT and MinWMT, respectively, demonstrates that the relative change on water temperature occurs primarily through the associated impact of increase/decrease of the water column depth, which is also connected to the increased/decreased thermal inertia of the river.

Updating the cross-sections in the study area, in addition to increasing the Manning’s roughness (Scenario 2), caused an increase of 17 cm in the average flow depth compared to the baseline case (from 26 to 43 cm), and a 20% decrease in the average velocity (from 0.79 m/s cm to 0.63 m/s cm; Figure 3; Supplementary Table S4). The increase of flow depth starting at station 7,500 m for the next 1 km was more pronounced by about 40 cm. This increase has the potential to provide thermal refuge for the steelhead or sucker, however, the average MaxWAT in this reach was 28.1 C so we were not able to support this claim. Executing Scenario 2 demonstrated the same pattern of changes in the average MaxWMT and MinWMT that was noted for Scenario 1 (Table 2). The difference between the average MaxWMT and MinWMT for Scenario 2 was 12.9°C, which was 37% lower than the same difference for the base case and 11% lower than Scenario 1. A smaller difference between these thermal factors under Scenario 2 indicates that deeper in the water column caused fewer fluctuations in the diel river water temperatures as reported by Gu et al. (1999). The average MaxWMT for Scenario 2 dropped to 30.4°C, demonstrating a 9 and 2.2% decrease compared to the baseline case and Scenario 1, respectively. The average MinWMT for Scenario 2 increased to 17.5°C which was 36.7 and 5.4% higher than the base case and Scenario 1, respectively (see Figure 4). The average MaxWAT also was 28.3°C which, even though it was 0.6°C lower than the base case condition similar to Scenario 1, was higher than the considered steelhead survival line (by 3.3°C; Table 2).

Under Scenario 3, including groundwater upwelling in the study area, the average MaxWMT and MaxWAT values for the simulation period were 30.1°C and 28°C (Table 2). Although Scenario 3 provided the lowest values for the higher ranges of the determined thermal metrics, they were still higher than the desired temperature range for the steelhead migration and resident sucker (Figure 4; Table 2). Subsurface inflow temperature and volume parameters are typically sensitive parameters that affect river water temperature significantly (Abdi and Endreny, 2019; Abdi et al., 2020b). By studying the Shasta River tributary in northern California, Nichols et al. (2014) showed that subsurface upwelling inflows could act as reservoir releases to decrease water temperature. Loheide and Gorelick (2006) noted the importance of hyporheic exchange, as an important form of subsurface inflows in Cottonwood Creek in northern California during summer dry weather. They demonstrated that the absence of the subsurface inflows could lead to river temperatures warming in the downstream direction. In winter, Risley et al. (2010) and Kurylyk et al. (2016) documented that groundwater temperature could also contribute a warming effect where the river water temperature is typically below subsurface water temperatures. During the summer, in the LAR, Abdi et al. (2020a) simulated a similar phenomenon of groundwater cooling by redirecting warm surface inflows to infiltration via constructed riffles and pools, which then entered the river as cooler groundwater inflows. These findings indicate how pronounced the subsurface inflows are for reaching determined thermal thresholds for steelhead migration feasibility.

Other studies have demonstrated the impact of flow change on water temperature due to the hydraulics of river flow (Gu et al., 1999; Sinokrat and Gulliver, 2010; Abdi B. et al., 2021). Gu and Li (2002) showed that river water temperature’s sensitivity to flow and its properties (in first 20% flow change) is as significant as that to climate data such as air temperature, humidity, and solar radiation. Hockey et al. (1982) studied the variation of water temperature in the Hurunui River in New Zealand and found that for every 1 m³/s reduction in river flow, water temperature increased by 0.1°C. Garner et al. (2017) studied a 1,050 m reach of Girnock Burn River basin in east Scotland and found that for the scenarios with low gradient velocities (0.023 m/s), the water stayed longer within the reach, causing higher maximum and lower minimum temperatures. The findings from Garner et al. (2017) support our results showing the effect of lower water velocities on simulated water temperature due to enhanced heat accumulation and dissipation. However, since we didn’t increase the canopy density in our scenarios, we didn’t get significant changes in water temperatures (Figure 2). Flow velocities in even lower gradients could cause a warming effect in the water temperatures as well, where the residence time would be too high which allows water more time to heat up from solar radiation. Therefore, depending on other factors (riparian shading, upwelling, and inflows), a threshold should be considered for the residence time in the rivers to avoid unwanted warming of the water. Comparing the findings of other studies and our results shows a mixed, sensitive, and uncertain response of water temperature to variation of flow depth and velocity. Therefore, water temperature needs to be considered as a limiting factor based on the results from our simulations and the overall uncertainty of water temperature’s response to changes in depth and velocity.

Ecological restoration scenarios such as riparian shading from tree canopy, cooler substrate temperature (Trimmel et al., 2018; Abdi R. et al., 2021), as well as additional groundwater recharge and hyporheic exchange inflow could be considered to decrease water temperatures (Saha et al., 2017). Sun et al. (2015) simulated river temperature along six separate reaches of Mercer Creek in Washington State and found that tree and hillslope shading reduced the annual maximum temperatures by 4°C. Further, Dbouk (2017) noted that conductive heat transfer approaches and embedding conduit materials with a high thermal conductivity into substrate materials that have a much lower thermal conductivity could be used to act as cooling channels. For subsurface inflows, as suggested by Abdi and Endreny (2019), the relative contribution of groundwater and hyporheic exchange inflow with river water varies by site conditions and seasonality. For the LAR, Abdi et al. (2020a) showed that an 18% groundwater inflow contribution during dry weather in only a 0.5 km reach decreased LAR water temperature by 0.3°C. The need for additional restoration actions will only be exacerbated by climate change (Justice et al., 2017; Merriam and Petty, 2019), which is projected to increase river temperatures in the West (Risley et al., 2010; Rheinheimer et al., 2015).

The baseline temperature conditions of the LAR are not cool enough to support the native Santa Ana sucker nor migrating steelhead. The trout have a thermal maxima of 25°C (Myrick and Cech, 2000; A. Myrick and Cech, 2005) and the sucker’s maxima has been observed to be 22°C (Moyle, 2002). Temperatures at or near the steelhead’s maxima have been observed to be a source of chronic stress in other southern California streams, which may make them vulnerable to other poor water quality conditions and physiological stress (Materna, 2001; Dagit et al., 2009). Warm stream temperatures will also block the migration of salmonids, temperatures above 23°C have prevented steelhead migration in the Northwestern United States (McCullough et al., 2001). The results of our temperature modeling demonstrate that the three restoration scenarios selected for this study do not improve thermal conditions enough in the LAR to support the return of these fishes.

Despite a generally thermally inhospitable environment, thermal refugia can occur in association with cold water patches created from tributaries, groundwater seeps and springs, and shade. Trout have been observed to occupy thermal refugia in streams that exceeded their thermal tolerance where the refugia were 3–8°C colder than ambient stream temperatures (Ebersole et al., 2001). Thermal refugia could be present throughout the LAR, providing a reprieve from warm stream temperatures which would allow trout and sucker populations to survive. However, areas for thermal refugia may be limited due to the primarily engineered nature of the channel. The temperature modeling conducted here did not evaluate fine-scale micro-habitats, instead, the stream temperature was modeled on a reach-scale. Our modeling results suggest that overall, the stream temperatures are not hospitable to the steelhead and Santa Ana sucker even in consideration of proposed restoration alternatives. Support of suitable habitat for resident and migratory fish will require additional measures to ameliorate thermal conditions and allow fish to reach upstream areas with more suitable physical and thermal habitats.

This work may be indicative of other urban rivers, in which restoration alternatives have been proposed to restore physical river parameters but water quality is not explicitly modeled or considered (Wohl et al., 2015). In addition to temperature, cold water fish are sensitive to other pollutants, like metals (Ingersoll and Mebane, 2014; Naddy et al., 2015) and trace organics contaminants (Petrovic et al., 2002), such as those from tire wear (Tian et al., 2021). Water quality stressors, in addition to changing water management regimes and the effects of climate change, should be studied in tandem with optimal hydraulic and temperature parameters for target species. Our work suggests that future studies and management recommendations should consider environmental conditions that are holistically needed to support target species. Further, this work serves as an illustration of the challenge of habitat restoration in urban rivers, given the uncertain climate future.

Like most other urban rivers, the LAR is severely impacted by anthropogenic development and urban activities and since it has been channelized and confined, it suffers from a decline in biological habitat and species diversity. Hydraulic conditions in the LAR channel are not suitable for many of the native fish fauna because of shallow depths and high velocities. Several restoration scenarios have been suggested to provide increased flow complexity and habitat heterogeneity within this confined urban stream, such as increasing the roughness of the channel substrate and redesigning the cross-sectional channel area (USBR, 2019). Restoration scenarios could facilitate the migration of the steelhead in the river, specifically targeting the passage from the Pacific Ocean to the upstream Glendale Narrows soft bottom area and upper tributaries. However, previous work focused on improving the depth and velocity for the fish habitat suitability while the thermal condition of the river in the migration season was overlooked.

Our simulations of the baseline condition showed the river temperature was about 4°C higher than the determined threshold for fishes, therefore river temperature in the migration season would not support sustainable migrating steelhead or resident sucker populations despite suitable water column depths and average velocities. Hence, river temperature should be considered as a limiting factor for habitat suitability in facilitating steelhead migration plans in the LAR. Further, after applying the developed restoration scenarios in the study area, our simulations showed that even though the restoration scenarios decreased the 4°C thermal gap, they were still higher than the desired temperature range for the steelhead migration and the resident Santa Ana sucker (about 3°C after applying three considered scenarios combined). This indicates that additional ecological restoration actions, such as shading, should be considered and applied to further decrease the water temperature in the river passage during the migration season and to support year-round resident native fish such as the Santa Ana sucker.