Estimates of tributary inflow by basin or area and by surface water or groundwater are presented in this report and itemized by subreaches in tabular form. Total estimated average annual tributary inflow to the Colorado River between Hoover Dam and Mexico, excluding the measured tributaries, is 96,000 acre-ft or about 1% of the 7.5 million acre-ft/yr of Colorado River water apportioned to the States in the lower Colorado River basin. About 62% of the tributary inflow originates in Arizona, 30% in California, and 8% in Nevada. Tributary inflow is a small component in the water budget for the river. Most of the quantities of unmeasured tributary inflow were estimated in previous studies and were based on mean annual precipitation for 1931-60. Because mean annual precipitation for 1951-80 did not differ significantly from that of 1931-60, these tributary inflow estimates are assumed to be valid for use in 1984. Measured average annual runoff per unit drainage area on the Bill Williams River has remained the same. Surface water inflow from unmeasured tributaries is infrequent and is not captured in surface reservoirs in any of the States; it flows to the Colorado River gaging stations. Estimates of groundwater inflow to the Colorad River valley. Average annual runoff can be used in a water budget; although in wet years, runoff may be large enough to affect the calculation of consumptive use and to be estimated from hydrographs for the Colorado River valley are based on groundwater recharge estimates in the bordering areas, which have not significantly changed through time. In most areas adjacent to the Colorado River valley, groundwater pumpage is small and pumping has not significantly affected the quantity of groundwater discharged to the Colorado River valley. In some areas where groundwater pumpage exceeds the quantity of groundwater discharge and water levels have declined, the quantity of discharge probably has decreased and groundwater inflow to the Colorado River valley will eventually be reduced if not stopped completely. Groundwater discharged at springs below Hoover Dam is unused and flows directly to the Colorado River. (Lantz-PTT)