Urbanization is occurring rapidly in southeastern Asia where streams are increasingly pressured. We assessed the ecological integrity of Ampang River in Kuala Lumpur by comparing structural and functional measures between forested and urban sites. We assessed 4 forested, 1 intermediate (deforested, not channelized), and 5 urban, channelized sites. Urbanization altered substrate (concrete at urban sites), riparian vegetation, light, temperature, O2, conductivity, nutrients, and major ion levels, and simplified food webs. Invertebrate composition shifted from pollution-intolerant taxa at forested sites to tolerant taxa at urban sites. Richness in Surber samples was 56 species at forested sites and 27 taxa at urban sites. Basal food sources at forested sites were leaf litter and biofilm, whereas at urban sites they were a sediment mat comprising organic matter, cyanobacteria, iron bacteria, and algae. Organic matter biomass on ceramic tiles was greater at urban than forested sites, but chlorophyll a was not. Faunal abundance was greater in litter bags at urban (mean = 72.6 animals/bag, 68 chironomids) than at forested sites (27.9 animals/bag; 33 chironomids), but species richness was higher in forested sites. Food webs at forested sites were more complex and differed from those at urban sites, which lacked shredders, and grazers differed in taxonomic composition and diet between urban and forested sites. At forested sites, insect larvae grazed biofilm, whereas at urban sites, snails consumed the organic mat. Leaf decomposition was fastest at the most disturbed site indicating rapid microbial decomposition caused by increased temperatures (+4.5?C) and nutrients, and possibly increased physical breakdown caused by rapid flow carrying abrasive sediment and garbage. Urbanization has had severe impacts on ecosystem function in Ampang River that could be alleviated by preventing pollution, restoring riparian vegetation, and providing natural substrate. ? 2015 by The Society for Freshwater Science.