Reexamining what is gained and lost by taming rivers.
While many people think of dams in terms of large federal projects, such as Lookout Point (the reservoir with the largest maximum storage capacity in the Willamette River Basin at 477,700 acre-ft.) or Cougar Reservoir (the reservoir with the highest dam in the Basin at 519 ft. above stream level), a significant percentage of dams within the Basin are small private dams constructed primarily for purposes of irrigation.
Over 2.7 million acre-ft. of water can be stored behind the 371 dams in the Willamette River Basin: an amount equivalent to a foot of water covering over 37% of the surface of the Basin. The Tualatin River watershed has both the greatest number of dams (82) and the greatest concentration of dams within a watershed with one dam for every 8.6 miles of watershed area.
The first three dams in the Willamette River Basin were constructed in 1894 by the City of Portland for water supply purposes. While the construction of dams by local governments has proceeded over time at a low level, most federal and private dams were built between 1950 and 1980. Construction has waned since the peak in 1965-1970, and as dams have been implicated in the decline of salmon populations (through changes in river ecology and denial of fish passage) new dam construction has become controversial.
Communities along the Willamette River have attempted to control the movement of the river since the floods of 1860 and 1861. Large stone is placed in riprap, wing deflectors, and levees to redirect the flow of the river and to prevent banks from eroding. While such revetments make sense to property owners on the edge of the river because they protect land at a specific location, they deny the natural tendency of a river to erode and deposit sediments. Channel meandering is a natural process by which a river dissipates its energy during floods. Channel straightening and hardening of banks tends to increase the energy of the river during floods and potentially creates accelerated erosion at other locations.
Extent of Revetments
In the Willamette River, more than 96 miles of revetments have been constructed. Approximately half of the length of revetments was constructed by the U.S. Army Corps of Engineers. Revetments downstream of Newberg are not operated or constructed by the Corps of Engineers and were constructed between 1938 and 1968.
Though almost three quarters of the length of the Willamette River has no riprap or bank revetment on either bank, the degree of channel control is far greater than this statistic may imply. Changes in river channels occur at meander bends and side channels. Even though only 26% of the length has bank revetments, 65% of the meander bends are revetted. The majority of the dynamic sections of the river have been armored to eliminate or reduce change in channel form and position. This diminished ability of the channel to adjust its bed and sediment storage coupled with the active elimination of side channels has greatly simplified the river and diminished the complexity and abundance of aquatic habitats and has greatly reduced the ability of the river to interact with groundwater (known as the hyporheic flow).