7780

• UCREFRP Catalog Number: 7780
• Author: Brooks, J. E., et al.
• Title: San Juan River Seven Year Research Program Summary Report 1992.
• USFW Year: 1992.
• USFW - Doc Type: \
• Copyright Material: NO

geomorphic characterization. Areas of open water, secondary
channel, back water and isolated pool by river mile were included
in the analysis.
A cluster analysis was performed to identify groupings of the
channel conditions discussed above. The results of the analysis
indicated that there were not uniform geomorphically similar
reaches within the study area. Except in a few locations, factors
such as stream sinuosity, stream width, and the frequency of side
channels were fairly uniformly distributed in the study area.
While repeat patterns of some characteristics were identified, when
combined with all features analyzed, the patterns are not repeated
such that detailed studies of a shorter reach could be extrapolated
to the whole.
River Channel Dynamics - A preliminary historical analysis of the
river system depicts the river from the late 1800's to about 1940
as being broad and heavily sediment-laden with little channel
definition brought about by a surplus of sediment inflow, relative
to the transport capacity of the river. The most probable cause of
the condition of the river during this time is that of climate
change, associated with over-grazing. These conditions caused
severe erosion in the- watershed, with subsequent sediment
overloading in the San Juan River. Extreme flow events with flows
in excess of 2, 830 m3/sec (100, 000 cfs) near Bluff shortly after
the turn of the century probably contributed to the condition of
the river. Review of aerial photography from 1934 confirms the
condition of the river discussed in the historical documents.
In the 1940's the trend reversed; sediment inflow decreased and the
river degraded and channelized into the deposits left during the
preceding 60 or so years. By the 1950's aerial photography
confirmed the channelization and indicated the beginning of
vegetative encroachment in the previous flood plain. Comparing
1988 aerial photography with photography from the 1930'x, 50's and
60's indicated that the channelization, especially in the lower
reaches of the river, continues today, but at a slower rate.
Analysis of historic sediment loads in the San Juan River indicated
a major shift toward .reduced sediment load in the 1940' s . This
corresponds with the aerial photo interpretation and a condition of
reduced sediment production in the watershed, possibly due to
another climate shift. Another shift downward in the sediment
production curve occurred in 1973. The cause in the shift is
unknown, unless it is related to a delayed response from the
operation of Navajo dam as the sediment load and transport capacity
reached a new equilibrium.
An additional contribution to channelization was the encroachment
of tamarisk and Russian olive in the basin. As the river began to
channelize the invasion of these species along the more stable
river banks increased channel containment and provided some
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