Tributary confluences are dynamic thermal refuges for a juvenile salmonid in a warming river network

As rivers warm, cold-water fish species may alleviate thermal stress by
moving into localized thermal refuges such as cold-water plumes created by
cool tributary inflows. We quantified use of two tributary confluence
plumes by juvenile steelhead, Oncorhynchus mykiss, throughout the summer,
including how trout positioned themselves in relation to temperature
within confluence plumes. At two confluences, Cedar and Elder creeks,
along the South Fork Eel River, California, USA, we monitored temperatures
using in-situ logger grids throughout summer 2016. Fish were counted
within confluences via snorkel surveys five times a day on five days at
each site. We found diel and seasonal dependence on confluence use by
steelhead, especially at the Cedar Creek confluence, where mainstem
temperatures exceeded 28°C. At this site, fish moved into the confluence
on the warmest days and warmest times of the day. Fish observed within the
Cedar Creek confluence plume were most common in locations between
20-22°C, rather than the coldest locations (14.5˚C). At Elder Creek, where
mainstem temperatures remained below 24˚C, there was little relationship
between mainstem temperature and steelhead presence in the confluence
plume. At both sites, steelhead distribution within plumes was influenced
by spatial variation of temperature and mean temperature in surveyed grid
cells. Our results show that cool tributaries flowing into warmer
mainstems reaches (over 24˚C) likely create important thermal refuges for
juvenile steelhead. As mainstem rivers warm with climate change,
cool-water tributary inputs may become more important for sustaining
cold-water salmonids near the southern end of their range.