Population changes in Chironomids in 2 sets of water spreading basins and a flood control channel in the Santa Ana River basin, southern Calif., were studied from June 1974 to June 1975. Benthic samples from 6 basins (3 of each set) and 4 ponds in the channel, when flooded, were collected biweekly. Water temperature at the time of sampling was recorded. Larvae of 11 genera were collected. Tanytarsus spp., Chironomus spp., and Procladius spp., in that order, were quantitatively important. Tanytarsus spp. predominated in the area from Aug, 1974 to Apr, 1975. Chironomus spp. prevailed in large numbers during Oct-Nov, and Apr-June. Procladius spp. was abundant from June-Sept. The mean density of total larvae in one set of basins was ca. 2000/0/09 m2 (=ft2 during Oct-Nov. 1974 and in May 1975. In the channel ponds the mean density was 1470/0.9 m2 in Oct-Nov and 3000/0/09 m2 in May-June. These 2 periods seem to experience heavy midge outbreaks. Intermittent drying and flooding of spreading structures affected the abundance of midges. Areas flooded for long periods and supporting smaller midge numbers prior to drying, invariably supported higher densities when renovated and reflooded. Sharp increases in midge larval densities occurred with-in 2-4 wk after flooding. An increased frequency of drying and flooding of the spreading structures adds to the midge nuisance problem in the vicinity of the water spreading system along the Santa Ana River.