Techniques to measure discharge and nutrient loads in the tidally affected portions of two major rivers tributary to Tampa Bay, the Alafia River and the Hillsborough River, were developed and tested. Discharge, water quality, and total phosphorus and total nitrogen loads for the period April 1, 1991, through March 31, 1992, were evaluated and compared with discharge,water quality, and loads at long-term, nontidal gages in the basins. Long-term discharge and water-quality characteristics at selected sites in the Alafia river and Hillsborough River basins were evaluated. A long-term, decreasing trend in annual-mean discharge was observed for discharges at the Alafia River, Sulphur Springs, and Hillsborough River. Low-flow and high-flow characteristics in the Alafia River and Hillsborough River have changed as well. The decreasing trend in the Alafia River discharges is not due to deficient rainfall but probably is due to decreased ground-water inflow to the river because of long-term declines in the potentiometric surface of the Upper Floridan aquifer. Daily-mean discharges at the mouth of the Alafia River were more variable than discharges at the long-term gage upstream. Daily-mean discharge near the mouth of the river was negative at times, indicating a net loss of water from the river. Daily-mean discharge from the Hillsborough River was minimal from Apil to May 1991, and from late September 1991 to March 1992. During these periods, discharge from Sulphur Springs was a major source of freshwater to the tidally affected reach of the river. Concentrations of total phosphorus and orthophosphorus in the Alafia River above Lithia Springs were the greatest in the 1960's and have generally declined since then. Total nitrogen concentrations have been declining since about 1981. However, increases in nitrate plus nitrite nitrogen concentrations are occurring in Lithia Springs, a second-magnitude spring that flows into the Alafia River. Specific conductance of water discharging from Sulphur Springs to the Hillsborough River has increased from about 124 to more than 2,000 microsiemens per centimeter since 1945. Water quality at the mouth of the Alafia River and Hillsborough River is the result of mixing of freshwater and estuarine water from Hillsborough Bay. Large daily variations in water quality occur at these site because of tidal currents, and vertical stratification of specific conductance is a common feature. Concentrations of phosphorus, nitrate plus nitrite nitrogen, organic carbon, and silica are inversely related to specific conductance at the mouth of the Alafia River. Constituent concentration and discharge data were used to compute loads during the study period. Average daily phosphorus loads were 2.4 tons per day at the mouth of the Alafia River; 0.35 ton per day at the mouth of the Hillsborough River; and 0.06 ton per day at the Tampa Bypass Canal. Average daily nitrogen loads were 1.7 tons per day at the mouth of the Alafia River; 0.86 ton per day at the mouth of the Hillsborough River; and 0.26 ton per day at the Tampa Bypass Canal. The greatest annual loads of phosphorus and nitrogen from the major tributaries to Hillsborough Bay are from the Alafia River, with the greatest loads at the river mouth. Total phosphorus load from the Alafia River was about 894 tons during April 1991 through March 1992, more than six times greater than phosphorus loads form the Hillsborough River. Annual total nitrogen load at the mouth of the Alafia River was about 630 tons, two times greater than at the mouth of the Hillsborough River and more than six times greater than loads from the Tampa Bypass Canal. Basinwide yields of total phosphorus during April 1991 through March 1992 were about 2 tons per square mile at the mouth of the Alafia river basin and were about 0.2 ton per square mile at the mouth of the Hillsborough River. Total nitrogen yield was about 1.5 tons per square mile at the mouth of the Alafia River and about 0.5 ton per s