Fifty-four major streams discharging directly to the Gulf of Mexico and having drainage areas exceeding 200 square miles were identified in the United States. Forty-four U.S. Geological Survey streamflow-gaging stations along the Gulf of Mexico with at least 40 years of daily streamflow data also were identified. These stations include most of the major streams and comprise 95 percent of the drainage area to the Gulf from the United States.

Daily mean values of continuously monitored streamflow were aggregated, annually and monthly, for selected stations. The mean, minimum, and maximum values were determined for the aggregated data for each station.

Statistical and graphical representations of temporal trends in streamflow are given for stations included in this report. Substantial percentage changes in annual and monthly streamflow between early (before 1963) and late (1963-90) time periods describe long-term temporal trends in streamflow for most of the 44 long-term stations. Graphical representations of long- and short-term temporal trends are presented for total gaged annual mean streamflow for each of 12 segments dividing the Gulf Coast.

Temporal trends in streamflow were related to major factors that affect streamflow: precipitation, land use, withdrawals of surface water, reservoir operations, and other factors. Low- and highstreamflow periods are related to extremes in precipitation; substantial increases in streamflow are associated with urbanization; decreases in streamflow are coincident with increases in withdrawals of surface water; and increases in minimum streamflows and decreases in maximum streamflows are associated with increases in the number of reservoirs. Other factors (springflow, soil composition, and effluent discharges) affect monthly minimum streamflow for some stations.

Seasonal or monthly distributions of streamflow were determined for each of the 44 long-term stations; months with high or low streamflow are presented for each station. Precipitation is the primary factor that affects the distributions of streamflow. Other factors that affect streamflow distributions include land use in urban areas, withdrawals of surface water for irrigation, and reservoir operation.