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l thread channel. This change has potentially serious consequences for the native salmon population. During high flows on the McKen- zie, the river exerts a high boundary shear stress (the force per unit area exerted by flowing water on the streambed, which is responsible for moving sediment; Figure 4), and consequently the bed material of the river tends to be quite coarse: cobbles and boulders, with a median par- ticle size of approximately 200-300 mm-too coarse for salmon to spawn in. Given the steep channel, smaller cobbles of suitable size for salmon spawning, approximately 100-150 mm, tend to accumulate at the sur- face only where topographic com- plexities lead to areas of reduced boundary shear stress, as in the lee of islands or in smaller side chan- nels. Midchannel bars develop when a stream deposits sediment that is lo- cally in excess of its capacity to carry it. Diversion of flow around the bar further reduces the local boundary shear stress and causes more deposition, which eventually leads to island formation (Leopold et al. 1964). The sediment that forms the midchannel bars includes coarse cobble/boulder glacial outwash ma- terial, derived primarily from the floodplain and terraces as a result of bank erosion and channel avulsions (the river cutting across the flood- plain at high flows, forming an en- tirely new channel path). The reduc- tion in peak flows, however, does not simply keep the water off the floodplain, it stabilizes the channel itself. The channel no longer avulses, nor actively cuts against its banks and terraces, thus eliminating, for the most part, the recruitment of coarse sediment. (Some bank pro- tection measures also reduce bank erosion.) Thus, reduction in the high- est peak flows prevents the creation of new midchannel bars and islands. The existing islands are gradually being lost as the smallest channels are filled in, and so the river is becoming simplified to a single- thread channel (Figure 5; Table 1). As the braided reaches disappear, the areas where spawning gravels can be deposited are being lost. In addition, the reduction in channel movement reduces the recruitment Figure 3. Single-thread channels in the McKenzie River are separated by stretches with islands and multiple channels. Bottom map shows expanded view of part of the top map. of spawning gravel from the flood- plain. The channel simplification is also affecting rearing habitat for juvenile salmon: sloughs, backwa- ters, and traces of former channels created by river avulsion and mean- der cutoffs are disappearing. There is evidence that spawning- gravel limitations are the source of the density-dependent mortality that determines the equilibrium or aver- age population size of the McKenzie River salmon. Estimates of spawn- ing-gravel availability indicate that even in years with the fewest return- ing spawners, there is inadequate March 1995 185