Importance of streamside forests to large rivers: the isolation of the Willamette River, Oregon, U.S.A., from its floodplain by snagging and streamside forest removal

The river continuum concept (VANNOTE et al. 1980) stressed the point that the influence of theterrestrial system on a stream diminishes as the stream gets larger. The concept argued for greaterautochthonous carbon inputs and lower tributary inputs of carbon to higher order streams (7-9thorders). The role of floodplains in the river continuum concept was limited to decomposition ofparticulate organic material during periods of low water and the subsequent return of organic ma-terials by flood waters and surface runoff. This floodplain interaction existed naturally on all rivers,including lowland streams and very large rivers (9-12th orders).
The river continuum concept (VANNOTE et al. 1980) emphasizes some functions of streamsideforest in inferring a downstream decrease in influence, but does not give attention to other func-tions related to overbank flow that increase in importance downstream as outlined by WELCOMME(1979). WELCOMME (1979) examined the great floodplain rivers of the tropics and argued for a tightcoupling of the floodplain with the river system. His basic premise is that the productivity of flood-plain fisheries was dependent on the extent and duration of flooding. WissmAR et al. (1980) reportedthat inorganic and organic materials of terrestrial origin probably influenced the microbial activityof the mainstem Amazon River, its tributaries, and its varzea (floodplain) lakes.
The relationship of floodplain and mainstem in large rivers in North America and Europe nolonger exists and is rapidly disappearing in Africa, South America, and Asia. The influence of thefloodplains has been reduced by (1) the extent of local activities such as snagging the mainstem, dik-ing, and improved drainage of floodplains for agriculture or urbanization; and (2) reduction of theextent of flooding because of upstream activities such as flood control dams. These alterationswithin the stream and on the floodplain have modified the relationship between mainstem andfloodplain by changing the composition and structure of the floodplain vegetation and changing thesources and sinks for organic matter along large rivers. The combined effects isolate a river systemfrom the influence its floodplain has on the structure and nutrient capital of the aquatic ecosystem.
The river continuum concept (VANNOTE et al. 1980; WELCOMME 1979) hardly mentions the roleof downed trees in large rivers. The ecological role of wood in large rivers will never be completelyunderstood because wood has been systematically removed along most rivers around the world fortimber, firewood, or as obstructions to ship and barge navigation (SEDELL & LUCHESSA 1982).
SEDELL et al. (1982) document snags (downed trees) pulled from many rivers in virtually everyregion of the United States of America. Stream improvement for navigation was well financed bythe United States government from 1870 to the present. Over 800,000 snags were pulled in a 50 yearperiod along the lower 1600 km of the Mississippi River. The snags were primarily cottonwood andsycamore averaging 1.7 m in diameter at the base and 0.7 m at the top and had an average length of35 m (SEDELL et al. 1982).
These snags created shoals, dammed sloughs, caused large side jams, and often completelyjammed a lowland river. The historical records of streamside forests are the only evidence re-maining of the importance of downed trees to channel geomorphology in pristine rivers.
This report describes the pristine and present streamside forest, channel geomorphology, and role of downed trees in the Willamette River, Oregon, U. S. A. From this case history, a modifica-tion of the river continuum concept is presented.