Data collection for the lower Illinois River Basin (LIRB) National Water-Quality Assessment (NAWQA) program began in 1996. Data on habitat, fish, benthic macroinvertebrates, and sediment were collected at eight stations on six streams in the basin--Illinois River, Panther Creek, Mackinaw River, Indian Creek, Sangamon River, and La Moine River. These streams typically flow through agricultural lands with very low gradients. Substrates typically are clay to gravel with areas of cobble. Banks are high, steep, and sparsely vegetated. Topographic surveys provide illustrations of the geometry that promote understanding of channel geometry and a data set that, in the future, can be used by others to assess stream changes. Suspended-sediment particle size, woody debris, and stream velocity are important to fish and benthic macroinvertebrate communities. Fine particles (silts and clays) were abundant in suspended sediment and stream banks, and fish insectivorous cyprinid community composition increased with decreases in the concentration of these suspended fines. Suckers were prevalent in stream reaches with abundant woody-snag cover, whereas sunfish communities were most abundant in areas with slow water velocities. Hydropsychidae, Chironomidae, and Baetidae were the most abundant benthic macroinvertebrate families collected throughout the region, but stream size and water velocity were important to benthic macroinvertebrate community composition. Tricorythodes mayflies and Elmidae had higher relative abundance at sites in small- and moderate-size drainage basins, and Baetidae density was greatest in reaches with highest water velocity.