There are 26 species of native and introduced fish in the lower Central Valley portion of the river, including Sacramento sucker, smallmouth bass, largemouth bass and carp. Anadromous fish species found in the lower river are chinook salmon, Pacific lamprey and striped bass. The upper section of the river, defined as the stretch from Lake McClure to the headwaters, has 11 species of fish. Historically, the range of anadromous fish extended to the waterfalls at the head of Yosemite Valley, but dams have blocked their migration since the early 1900s, and diversions frequently dewater their remaining spawning habitat in the lower part of the river. Environmental measures enacted in the late 20th century have had some success in boosting chinook salmon populations, from a record low of 500 fish during several years in the 1950s, to a high of 30,000 in 1984. Since the 1970s, the annual chinook run has averaged about 5,300.