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According to local landowners, channellzatlon of the streambeds in the Blacksmith Fork and Logan Rivers began over 50 years ago when the ranchers shovelled the streambed gravel by hand and transported this gravel from the stream by horse-drawn wagons. Short reaches of the rivers have been altered ever since that time. Today short reaches of the rivers are altered period- ically with backhoes and bulldozers by individual ranchers because of flooding in agricultural areas, and for flood control by the U. S. Army Corps of Engi- neers. Five study sites were selected that would provide information on three types of stream channel alterations (dredged, recently bulldozed, and old bull- dozed) and two control areas in the floodplains of these rivers (Figure 1). Erosion and deposition of streambed materials were directly correlated with the length of stream that was altered and the type of alteration. In addition, alterations that are made upstream can have a significant effect on scouring and filling in downstream reaches. The energy to move streambed materials depends upon the discharge at peak runoff; during extreme runoff, more and larger bedstream materials are rearranged as the water rises and then subsides. The amount of streambed materials that are rearranged even under the mild runoff that occurred in 1975-76 can be quite large, especially in newly altered reaches (Figure 2). The shape and integrity of the streambanks differed distinctly between dredging and bulldozing alterations. For example, alter- ations from bulldozing resulted in a uniform cross-section of the stream that made the stream wider and shallower, and the riparian vegetation along the streambanks was eliminated either by direct removal or by being covered with streambed material. In contrast, the banks were usually only altered on one side of the stream by dredging with a backhoe. In addition, the stream meander was not changed significantly by dredging while the stream was realigned into a straight reach by bulldozing. Fewer pools remained in reaches that were altered by bulldozer than in reaches that were altered by dredging. Generally, the gradient of the stream was made more uniform by bulldozing than by dredging and these changes persisted longer. High spring streamflows appeared to be required to restore the frequency and depth of pools in altered reaches. However, the degree of the alteration was also important. For example, alter- ations in the pool-riffle ratios and gradient by dredging generally reverted to the original character after spring runoff even during the gradual runoff that occurred during 1975-76. Most alterations to short reaches of stream channels in these two Intermountain streams were relatively short-lived. . However, numerous alterations made fairly frequently have resulted in unstable .~ streambeds and, consequently, are having a significant, detrimental impact on ] the trout populations in these streams. Because the various life stages of fish require different habitat condi- tions, the population densities varied widely from season to season even within a study site. In addition, age and size of the fish influence the behavior to select certain environmental conditions. For these reasons, direct comparisons of population densities is difficult. The overall changes in the population of brown trout (1973 year class) in the study sites is summarized in Figure 3. Alteration of the stream channel, even short reaches, by dredging and bull- dozing adversely affected the trout populations in these reaches. Much varia- tion occurred in the mountain whitefish populations at all sites (Figure 4 and 5). However, the whitefish populations were most stable throughout the year in ~ L