Abstract:
The Elk River Basin drains 93 sq ml of steep forested terrain
on the west side of the Klamath Mountains in Southwest Oregon.
This river and its tributaries support a diverse and abundant
population of anadromous fish; a hatchery located at river mile 13
(km 21) supplements these native populations. Clear weather in
combination with dry summers and low streamflow produce warm water
temperatures. Historical data indicate that summer stream
temperatures as high as 76°F (24.4°C) have occurred in the
mainstem of the Elk River.
This study was undertaken to evaluate summer stream temperatures
and to what extent they were affected by natural factors
and land use. In selected tributaries and along the mainstem
of the Elk River, measurements were made on canopy cover, stream
surface width, and thalweg depth during the summers of 1984 and
1985. Recording thermographs and maximum/minimum thermometers
were used to determine the overall temperature pattern of the
basin.
In genera.l, maximum stream temperatures appear to be
declining since 1970, following a period of increased landslide
activity in the basin. The trend of decreasing maximum
temperatures was not associated with changes in summer streamflow
or summer precipitation patterns, suggesting that a recovery of
streamside vegetation and/or change in channel morphology is
proceeding within the basin.
Tributaries with relatively large width / depth ratios (exceeding
14 or more), generally exhibited large diurnal
fluctuations in temperature indicating solar heating was
occurring. For drainages of similar size, large diurnal temperature
fluctuations were associated with large surface area to
volume ratios. Tributaries found to be especially low in maximum
temperatures generally flow subsurface as a result of channel
aggradation. In the 1984 and 1985 field seasons, maximum stream
temperatures in the tributaries never exceeded the maximum temperature
of 69°F (20.6°C) found in the mainstem. The upper
reaches of the mainstem, which is wide and aggraded, was the
portion of the basin found to have the highest maximum stream
temperatures.
Five pools were sampled to determine vertical profiles of
temperature and dissolved oxygen. Temperatures and dissolved
oxygen concentrations did not vary with depth.
