Historical change in channel form and riparian vegetation of the McKenzie River, Oregon

by Minear, Paula J.

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.