The role of near-channel hyporheic exchange on the thermal variation in a semi-arid alluvial river. A.S. Arrigoni. Masters Thesis. The University of California, Santa Barbara. 2004.
The Umatilla River in northeastern Oregon, USA once supported healthy
populations of salmonids, but currently summertime water temperatures
are exceeding 26ºC and are stressful or lethal. Loss of riparian shade
is generally identified as the dominant anthropogenic influence on
stream temperature, the summertime base flow channel of the Umatilla
River may have never been well shaded due to scouring of the bank-full
channel margins during seasonal floods driven by snowmelt. However,
upwelling groundwater is known to be a source of cooler water that
could provide refuge for fish during high summertime temperatures. In
this project we are documenting the influence of near-channel hyporheic
exchange on the river’s thermal regime. Three study sites incorporating
a human engineered channel and two geomorphically complex channels were
instrumented with 80 temperature loggers. These loggers were used to
capture the water temperature profiles of down welling water (aquifer
recharge) and the beginning and upwelling water (aquifer discharge) at
the end of near-channel hyporheic flow pathways. The resulting data
suggest that near-channel hyporheic exchange has the ability to reduce
water temperatures by 4°C in the two geomorphically complex study
sites, whereas the human engineered channel had no thermal variation.
In addition to creating the expected localized patterns of thermal
diversity in the river channel near upwelling water, my data suggest
that the cumulative affect of geomorphically complex nodes within the
river could have the ability to buffer diel temperature variation in
the main flow of the river. Thus, when daily maximum temperature occurs
and fish are most stressed by water temperatures, hyporheic exchange
creates cool pockets of water and appears to reduce peak temperatures
throughout the study sites. My results suggest that loss of geomorphic
complexity within the river due to flood-plain development, large wood
removal, and channel engineering may reduce the effect of hyporheic
flow as an important temperature regulation mechanism throughout the
river.
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