Currently, the Beaver River and its tributaries flow with water intermittently, in part because the underground source of the river, the Ogallala Aquifer, being the water table beneath far western Oklahoma and parts of seven other Western states, as well, has been subject to depletion in recent decades due to increased irrigation and drinking-water withdrawals. Water flow in the Beaver at Guymon, in the years prior to the start of construction on the Optima Lake dam (1937–1966) averaged 32.2 ft3/s, but only 7 ft3/s in the decade (1977-1987) after the dam was built. In the last five years (1989–1993 inclusive) that the U.S. Geological Survey could measure its current near Guymon, the Beaver's flow averaged less than one-fifth of 1 ft3/s. A few short stretches, such as McNees Crossing (one mile east of New Mexico State Road 406), have persistent flow. The remainder flows under the sand except after rain falls or snow melts. The area drained by the Beaver River in the high plains of northeastern New Mexico and the Oklahoma and Texas panhandles receives on average less than of rain annually.