Hyatt, T., 2021. A LiDAR-based assessment of riparian zones for the Skagit River (WA) watershed. Skagit River System Cooperative, La Conner, WA. pp. 25.

In the Skagit River watershed, as with most watersheds in the Pacific Northwest, it has
been known for some time that the quality and extent of riparian forests is a primary
driver of population dynamics for salmon and trout, including several threatened and
endangered species (Hall et al. 2018, Quinn et al. 2018, SRSC and WDFW 2005). Skagit
steelhead, for instance, despite the myriad freshwater and marine influences, exhibit
density-dependent population characteristics that indicate juvenile rearing habitat as a
predominant limitation on adult spawner returns (Scheuerell et al. 2020). Riparian forests
affect that freshwater habitat in a number of ways: by shading and cooling streams, by
providing large wood that creates pools and other habitat features, by stabilizing eroding
banks, and by providing vegetative and macroinvertebrate inputs that drive the food
chain, among other effects (Naiman et al. 2000).
Sun, shade, and temperature
The presence or absence of riparian vegetation can have pronounced temperature effects
on rivers and streams (Seixas et al. 2018, Pollock et al. 2009, Brown and Krygier 1970).
For a given rate of net solar input, the change in temperature of a stream is directly
proportional to surface area and inversely proportional to discharge (Beschta et al. 1987,
Brown 1969). Often the highest solar input and the lowest discharges occur
simultaneously. Brown (1969) was among the first to model stream temperature energy
balances in Pacific Northwest forested streams, taking into account solar radiation,
evaporation, conduction, convection, and advection. Measured stream temperature values
were within 1 F of the modeled value more than 90% of the time. Net thermal (solar)
radiation was the predominant source of energy to these small Oregon streams, with
evaporation and convection accounting for less than 10% of the total energy exchange
(Brown 1969).
Streams fed by groundwater often display a more uniform temperature, both diurnally
and year round (Johnson 2004). Poole and Berman (2001) point out the importance of
hyporheic flow in regulating stream temperatures. Johnson (2004) showed that diurnal
fluctuations of stream temperature in a bedrock reach were much greater than
downstream in an alluvial reach, although mean daily temperatures were similar.
Johnson’s heat budget calculations showed that streams do not absorb large amounts of
heat from the air, but that air and water temperature respond to the same temporal
fluctuations in solar inputs. In a study of small streams near the limit of perennial flow,
Janisch et al. (2012) found a clear difference in temperature response between
intermittent streams with beds of coarse rock, as opposed to continuous streams with
fine-textured stream beds and upstream wetlands. Small, intermittent streams where the
water goes sub-surface were thermally unresponsive, due primarily to the interaction of
groundwater and hyporheic flow (Janisch et al. 2012).
Peak daily temperatures are usually achieved during the late afternoon, and minimums
just before dawn. Increased solar exposure not only leads to higher temperatures, but to