In this paper we use three-dimensional seismic attribute imaging and well data to reveal the previously unknown quantitative measures, directionality, and spatial locations of the Oligocene middle Frio fluvial channel systems within an area of 254 km2 (98 mi2) that covers two oil and gas fields in the Texas Gulf Coast Basin of the United States. The objective of this study is to apply quantitative seismic geomorphology techniques to quantify the morphometric parameters important to building predictive geologic models for fluvial reservoirs. Three categories of channel systems are differentiated based on their geomorphology, seismic signature, and the mode of transport. The first, category 1, includes channel systems of high-amplitude, moderate- to high-sinuosity, mixed load channels. Category 2 channel systems are high-amplitude, straight to low-sinuosity, bed load channels with both category 1 and 2 channels filled with coarse-grained sandstone deposits. Category 3 crevasse channel systems are low-amplitude, highly sinuous, suspended load channels filled with fine-grained deposits. These fluvial system categories were found to show unique morphometric characteristics such as channel width, meander belt width, and meander length. Analysis of the middle Frio channel systems imaged in the south Texas study area revealed a significant downstream decrease of channel belt width along the length of the channel belts. The creation of a quantitative morphometric database for the middle Frio fluvial reservoirs in the basin would be very useful for exploration and development purposes. The results of this study may have general applicability to the Texas Gulf Coast Basin and to similar fluvial reservoirs worldwide.