Evaluating the impact of substrate temperature on thermal habitat
suitability and ecological restoration in shallow urban rivers
Abstract
Managing river temperature in highly urbanized stream systems is
critical for maintaining aquatic ecosystems and associated beneficial
uses. Elevated river temperatures arise from warm surface inflows from
impervious areas, channelization, the absence of riparian forests, and
the lack of groundwater-surface water interactions. In the current work,
we utilize a mechanistic river temperature model, i-Tree Cool River, to
evaluate the cooling impacts of alternative ecological restoration
scenarios: a) shading effects of tree planting in riparian areas and b)
alternative streambed materials. The model was calibrated and validated
on a 4.2 km reach of the Compton Creek in the Los Angeles (LA) River
watershed, California, for low and high flow periods. The Arroyo Chub
and Stickleback were considered the target species for analyzing thermal
habitat suitability. River temperature simulations showed that like the
ambient air temperature. The thermal response of the river in high flow
periods was a function of upstream river temperature , where in low flow
periods river water temperature was most affected by the tested
restoration scenarios. Tree planting in the riparian corridor decreased
the median thermal metrics: Max Weekly Max, Max Weekly Average, and Min
Weekly Min Temperatures by an average of 3 ℃ (13%) to 20.4 ℃, 19.7 ℃,
and 17.8 ℃, respectively. Using limecrete as an alternative bed material
to the current concrete bottom decreased the median thermal metrics by
an average of 0.9 ℃ (4%) to 22.7 ℃, 22 ℃, and 19 ℃, respectively.
Combining the two scenarios decreased the river temperature metrics by
an average of 4 ℃ (18%) to 18.2 ℃. Besides riparian vegetation,
altering bed material is an impactful option in case of groundwater
contamination and if channelized urban corridors lack the substrate to
support vegetation. The use of ecological restoration scenarios resulted
in summertime temperatures were within the documented spawning
temperature thresholds and therefore temperature would not be a limiting
factor in the potential reintroduction of the Arroyo Chub and
Stickleback to Compton Creek. This tributary could be considered as a
potential refuge and improved fish habitat in the LA basin during low
flow periods.