By Larry R. Brown and Terry
M. Short
U.S. Geological Survey Water-Resources Investigations Report 99-4088
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Abstract
Four studies were done in the upper Merced River drainage in Yosemite
National Park and nearby areas from 1993 to 1996. First, monitoring studies
of benthic algae, benthic invertebrates, fish, and habitat were undertaken
at sites near Happy Isles and Pohono bridges from 1993 to 1995 as part
of the National Water-Quality Assessment Program of the U.S. Geological
Survey. Second, an ecological survey of benthic algae, benthic invertebrates,
fish, and habitat was done in the upper Merced River drainage in 1994.
Third, a special study of benthic algae, habitat, and water quality was
done in the reach of the Merced River within Yosemite Valley to deter-mine
whether human activities were having meas-urable effects on the ecosystem.
Fourth, baseline data on benthic algae, benthic invertebrates, and habitat
were collected in 1996 at four sites, two of which were undergoing extensive
streambank restoration activities. Comparisons of the baseline data with
future collections could be used to assess the effects of streambank restoration
on aquatic biota.
The general conclusion from these studies is that water quality in the
upper Merced River was very good from 1993–1996, despite high levels of
human activities in some areas. Fish communities did not appear to be a
useful indicator of habitat and water quality because of low species richness
and the apparent importance of physical barriers in determining species
distribu-tions. Measurements of fish densities and size-distributions might
be useful, but would be logistically difficult. Benthic algae and benthic
invertebrates do appear to be useful in monitoring environmental conditions.
Benthic algae may be more sensitive than benthic invertebrates to small
environmental differences within years. Benthic algae were also more responsive
than benthic invertebrates to differences in discharge between years. Thus,
benthic invertebrates may be more useful in comparing environmental conditions
between years, independent of discharge conditions.
