ponderosa pine is most common, dominate the landscape. The basin ranges from semi-desert to alpine climates, providing habitat for hundreds of species of plants. In the lowermost part of the watershed, in southeastern Washington, the Snake River is surrounded by an area called the Columbia Plateau Ecoprovince, which is now mostly occupied by irrigated farms. The rest of the Plateau area is characterized by low hills, dry lakes, and an arid, nearly desert climate. The headwaters of the Snake River and the high mountains elsewhere in the watershed were historically heavily forested. These include aspen, Douglas fir, and spruce fir, comprising about 20% of the historic watershed. At the base of mountains and in the Lost River basin, sagebrush was and is the predominant vegetation cover. Because of deforestation, up to one quarter of the forests have been taken over by sagebrush, leaving the remaining forests to cover about 15% of the watershed. However, the lodgepole pine has increased in number, taking over historic stands of other conifers. There are also up to 118 species of rare or endemic plants that occur in the Snake River watershed. Salmon and other anadromous fish The Snake River was once one of the most important rivers for the spawning of anadromous fish—which are hatched in the headwaters of rivers, live in the ocean for most of their lives, and return to the river to spawn—in the United States. The river supported species including chinook salmon, coho salmon, and sockeye salmon, as well as steelhead, white sturgeon, and Pacific lamprey. It is known that before the construction of dams on the river, there were three major chinook salmon runs in the Snake River; in the spring, summer and fall, totaling about 120,000 fish, and the sockeye salmon run was about 150,000. The historical barrier to fish migration on the Snake River was Shoshone Falls, a waterfall that occurs as the Snake River passes through the Snake River Plain. Since the early 20th century, when Swan Falls Dam was constructed on the middle Snake River upstream of Hells Canyon, the fifteen dams and reservoirs on the river have posed an increasing problem for migrating salmon. Agricultural lands and their resulting runoff have also had a significant impact on the success rate of migrating fish. Salmon can travel up the Snake River as far as Hells Canyon Dam, using the fish passage facilities of the four