The Ballena Gravels are the remnants of an ancient river system that flowed westward across the ancestral Peninsular Ranges during the Middle and Late Eocene epochs. The gravels are an eastern channelized fluvial facies of the Poway Conglomerate depositional system. The purpose of this study is to reconstruct the hydrologic conditions that prevailed during the deposition of the gravels. Sieve analyses show that the gravels are poorly sorted and have an average grain size of -5.5 phi. Measurements of large clast diameters show an apparent decrease in grain size from east to west. When these data are applied to Sternberg's logarithmic relationship, they suggest the distance to the "Poway rhyolite" source area was on the order of 75 to 125 miles. The slope of the ancient Ballena channel in the San Diego area is estimated to have been from 13 to 44 ft/mi. This estimate is based on an equation for the longitudinal profile of a stream with major variables of stream length, fall distance, and profile concavity. Estimates of stream velocity, based on a competent particle size of 52 cm, range from 6 to 13 ft/sec. The estimates are made using a multiple techniques analysis; seven techniques relate particle size to average stream velocity and four techniques relate shear stress to particle size. Eight different techniques, based on slope and velocity, are used to estimate channel depth. The values, which ranged from 8 to 15 feet, represent depths that occurred when the stream was at a high flood stage. Flow regimes in the Ballena River are estimated from the Reynolds and Froude equations. The equations suggest that lower flow regime conditions were prevalent and that upper flow regime conditions existed only when water depths were low (5 ft) and velocities high (13ft/sec). Sedimentary structures found in the gravels suggest that upper flow regime conditions prevailed much of the time. Paleodischarge is estimated using regime-type engineering equations based on slope, depth, and grain size. Discharge is also estimated using a technique based on stream length and drainage area. Discharge values generated from the six techniques have a strong overlap in the range from 200 to 10,000 ft3/sec. The magnitude and frequency of larger periodic flood discharges are estimated based on a logarithmic curve. Channel width values from 75 to 250 ft are also calculated from discharge using regime-type engineering equations. Ancient rainfall, based on runoff and temperature, is estimated to have been from 19 to 31 inches annually. Runoff is estimated from discharge values whereas paleotemperature is based on sedimentary features. The Nueces River of Texas may represent a modern analogue to the ancient Ballena River. The Nueces has a highly varied discharge similar to that interpreted for the Ballena River. Similarities also exist in sediment size, channel morphology, and flow characteristics. Several rivers flowing off the eastern side of the Andes Mountains in southern Argentina may also represent possible modern analogues.