| Abstract |
- This study examined channel structure and position and riparian
vegetation and land use on the upper 70 km of the McKenzie River, Oregon in
the 1940s, compared the 1940s conditions to present conditions, and explored
the processes driving change in this system and the implications for aquatic
habitat. The hydrologic record was analyzed, and field surveys were conducted
and compared to historical habitat surveys. Riparian characteristics and
channel features were digitized from aerial photographs from 1945/49 and
1986 and imported into Arcinfo GIS for analysis. Types of data digitized from
the aerial photos included locations and length or area of wetted channel,
active channel, tributaries, side channels, large woody debris, exposed gravel
bars, roads, and dominant vegetation or land use within 200 m of the active
channel.
Construction of dams on the mainstem Mckenzie River and two major
tributaries, Blue River and South Fork, in the 1960s has altered the flow regime
and sediment supply to the mainstem Mckenzie, decreasing the frequency,
mean and variation of peak flows, reducing the competence of flows to move
existing bedload, and cutting off sediment from over half of the drainage area.
Mean peak flows decreased 44% and competence of peak flows with a 2-yr
recurrence interval declined approximately 29% after dams were constructed
upriver. Adjustments to reduced sediment supply and flow alteration by dams in
this system included 57% decrease in exposed gravel bars, 40% decrease in
side channel length, and possible substrate coarsening (as compared to
historical estimates).
Channel straightening occurred in each of three instances of channel
change during the study period, and sinuosity decreased one half of the amount
needed to produce a straight channel in the most susceptible, unconstrained
reach. Human actions prior to high flow events played a role in the direction of
channel change in each case. Over the entire study area, 7% of the main
channel changed position by 30 m or more and little or no change in channel
position was noted in reaches constrained by valley floors. Additional channel
constraint has been produced by road construction near the channel and
riprapping for roads, bridges, and residences.
Less large woody debris was observed in the 1986 channel than in the
1949 channel, indicating a reduction in pool-forming agents and channel
roughness elements. Frequency of large pools ([greater than or equal to] 2 m depth and >40 m² area)
decreased 19% over the study area. The greatest loss in pools (73%) was
noted in the unconstrained reach that exhibited two areas of channel change
and an increase in exposed gravel bars.
Increased human use of the riparian area for roads and residential
purposes has led to an increased fragmentation of the riparian landscape.
Density of residential or developed patches within the riparian area has
increased 215% as more and smaller areas are converted from natural
vegetation to human use. Riparian area devoted to roads and residential uses
has nearly doubled since the 1940s. Mean vegetation or land-use patch size
has decreased from 2.2 ha to 1.6 ha as larger patches have been sub-divided,
and patch and edge densities have increased. Agriculture and clearcuts for timber removal have decreased within the riparian area while continuing
upslope. Riparian area in mature conifers has decreased 44% from levels in
the 1940s while hardwoods have increased 45% in the riparian area. Future
wood loading to the channel is reduced by a decline in mature riparian
vegetation, especially mature conifers.
Channel and riparian changes noted in this study have implications for
fish populations. Channel straightening, reduction in side channels, and loss of
pool-forming agents reduce habitat heterogeneity and off-channel refugia.
Ecosystem management of watersheds requires evaluation of conditions
across scales of time and space. The use of GIS in this study made it possible
to detect changes in channel form and riparian conditions during four decades,
along a 70-m channel and 90-m riparian area and to analyze the large data
sets relevant to understanding functions and change in channels and riparian
areas.
|
|---|
