Expanding bioenergy conversion and composting of organics can enable a near-term transition away from the landfilling, burning, and mismanagement of biomass residues. Strategic development of transportation, storage, and conversion infrastructure to enable this expansion requires detailed information on patterns and drivers of waste biomass production, quality, and geography that are currently lacking. This analysis contributes new geographic and temporal data on biomass residue availability for the state of California. Biomass residues are characterized for the year 2014 at the county- and month-scales for the agriculture, municipal, and forestry sectors in California, with values collected or estimated from numerous publications, databases, industry surveys, and methodologies. We present methods for developing supply scenarios out to the year 2050 that reflect anticipated changes in key environmental, market, and policy drivers. Our results suggest that biomass residue production could grow 16% by 2050 to 71 million tonnes of dry-matter per year, and that the co-processing of diverse high-moisture residue sources and storage of seasonally available low-moisture residues is needed to ensure adequate steady supply to bioenergy and composting facilities. Additional research and better reporting on organic waste management is needed to bound uncertainties regarding the response of residue production to market trends and recycling policies, and the influence of agricultural practices, plant selection, and climate impacts on residue yields.