Los Angeles County (LAC) is one of the largest urban population centers in the water-stressed Western United States. In this study, it serves as a testbed to explore how population growth, urban expansion and densification, and future water supply interact to influence water shortage outcomes. Our study uses a network flow optimization model, building off the existing Artes model of LAC’s water supply system, to interrogate how LAC’s water system responds to changes in future demand and supply. The model resolves over 70 water utilities that supply water to over 10 million people. The network has 342 nodes representing sources of supply or demand, and 717 links that define node topology and link-specific conveyance capacities, and runs at monthly resolution. A novel aspect of this work is that future decadal demands from 2030 to 2100 are generated from population and urban morphology evolution raster datasets for LAC associated with the shared socioeconomic pathways (SSP) scenarios. Population within each provider drives indoor water demand growth, while outdoor demand is projected using green fraction-urban land class relationships established using high-resolution landcover data for LAC. Presently, most of LAC’s water is supplied from a combination of local groundwater and imported surface water. LAC’s dependence on imported water exposes it to potential shortages during severe or prolonged drought conditions that reduce availability of imports. Due to deep uncertainty around future imports, we instead take a counterfactual approach where historical imports over a 25-year period (1986-2010) are scaled (increased or decreased) and applied to the future demand scenarios reflecting population growth and changes to urban morphology. The provider-level resolution of the model allows us to not only quantify the frequency and magnitude of shortages for LAC as a whole, but also to characterize provider-level vulnerability due to differences in provider supply sources (ex. Imports, reservoirs, groundwater, reuse) and projected urban morphology patterns.