Hawai‘i's reservoirs face increasing scrutiny due to heightened dam safety and flood control concerns, growing water demands, and uncertain water pollution effects. To promote long-term reservoir sustainability, it is vital that we improve our understanding of reservoir capacity. Because many of Hawai‘i's reservoirs are privately owned and unmonitored, the state of Hawai‘i lacks data on actual reservoir capacity. To conduct a statewide survey of reservoir capacity, we (1) collected, organized, and analyzed existing physical data characterizing reservoir capacity in the five main Hawaiian Islands; (2) interviewed reservoir managers throughout the state to assess current knowledge of reservoir capacity and its loss due to sedimentation; and (3) analyzed images to estimate available capacity and sedimentation. Results show that sedimentation is a serious concern for managers and is rarely measured and poorly documented. Approximately 79% of the total capacity of Hawai‘i's reservoirs are currently in use, and 40% of reservoirs (representing 24% of total capacity) are affected by sedimentation. Of these reservoirs, 60% are less than 1.23 million m³ (1,000 ac-ft) in size. Lack of maintenance for sediment is especially common in privately owned reservoirs. In the face of changing land use, reservoirs could be adapted for more appropriate uses in Hawai‘i, including flood control, hydropower, and recreation. Reservoir maintenance, including assessment and removal of sediments, is a critical consideration for proper stewardship of Hawai‘i's abundant water resources.