In the last several decades, we have understood that rivers and streams are products of their valleys and activities in the basins. However, thinking about aquatic habitat has lagged behind other areas. Our focus is still largely at the habitat unit (individual pool or riffle) or reach scale (about a 100 foot of stream length). The unifying process at the watershed scale is the movement of sediment and organic matter from the ridge tops, down hillsides, into tributary streams and then into the mainstem. I call it the digestive system of the basin. It is this movement of material through the basin which creates the food resources and aquatic habitat in the rivers. This movement is largely determined by the geology and climate of the basin; however, it is highly influenced by vegetation on the hillslopes and the valley floors.
The major organic matter (food resources) that drives the stream ecosystem originate mostly as leaf litter, needles and salmon carcasses, and to a lesser degree, algae. Originally, most of this material entered the streams during the fall and winter during storms. In studying the movement of this material in a coastal Oregon stream, most of the material entered directly as leaf fall or in landslides during the major winter storms. Large mature conifers provide the key pieces which stop and hold this material in the rivers long enough for the macroinvertebrates (mostly insects) to eat it. Healthy streams are highly connected to their floodplains and they are the most retentive of their organic matter. It is the insects which provide the link to salmon. In the study stream in Oregon, the coho salmon were food limited at 1-2% of their historic abundance, because the organic matter was not retained long enough to be eaten by the insects.
Restoring the natural movement of sediment and organic matter starts by protecting those portions of the watershed which are currently moving material in a natural manner. If we do not protect those areas, the basin will continue to degrade no matter how much money and effort we expend. We can then begin identifying areas adjacent to the healthy areas to begin to rebuild a more natural movement of material. In general areas closest to the stream, the riparian zones are the most critical. Restoration of aquatic resources takes a lot of time and must be based on a whole-basin perspective.
Charley Dewberry
Ecotrust, Portland, OR