This metadata record describes two metrics that quantitatively measure the impact of reservoir storage on every flowline in the NHDPlus version 2 data suite (NHDPlusV2) for the conterminous United States. These metrics are computed for every 10 years from 1800 - 2015. The first metric (DamIndex_EROM.zip) estimates reservoir storage intensity in units of days based on reservoir storage in a contributing area normalized by the mean annual streamflow. This metric indicates the duration of storage impact upstream from each stream segment relative to the typical flow condition. In addition, this metric provides an assessment of the potential influence of a dam on average and low flows because the metric estimates the number of days of flow that can be sustained by contributing area storage alone, without additional water or groundwater input. The second metric (DamIndex_PMC.zip) represents the degree of regulation of a river reach based on upstream reservoir storage relative to the 30-year average annual precipitation, as well as the upstream dam and watershed areas. This second metric provides an estimate of the capacity of the contributing area to store precipitation and is oriented to understanding how peak flows may be affected by dams throughout the flow network; this metric is dimensionless. Reservoir storage, construction date and location data were obtained from the US Army Corps of Engineers' National Inventory of Dams (NID, 2018). Also, the dataset in this data release includes dam locations addressed to NHDPlusv2 (Final_NID_2018.zip). These calculations are based on the maximum NID storage , which indicates the maximum amount of water that can be stored behind each dam and therefore may overestimate the true reservoir storage impacts.