Technical Abstract: Groundwater is a key driver of riparian condition on dryland rivers but is in high demand for municipal, industrial, and agricultural uses. Approaches are needed to guide decisions that balance human water needs while conserving riparian ecosystems. We developed a space-for-time substitution model that links groundwater change scenarios implemented within a Decision Support System (DSS) with proportions of floodplain vegetation types and abundances of breeding and migratory birds along the upper San Pedro River, Arizona. In groundwater decline scenarios, relative proportions of tall-canopied cottonwood/willow on the floodplain progressively decline, and shrubbier species less dependent on permanent water sources increase. These scenarios result in broad shifts in the composition of the breeding bird community, with canopy nesting and water-obligate birds declining but midstory nesting birds increasing in abundance as groundwater declines. Groundwater recharge scenarios were associated with increases in canopy nesting birds. Model outputs serve to assess the sensitivity of biotic groups to potential changes in groundwater and thus to rank scenarios based on their expected ecological impacts.