Water depth

Dam Builder

Draw a line across a valley to place a dam. The tool floods the terrain behind it and reports the reservoir and dam statistics.

The results are rough estimates from public data. Do not use them for engineering, flood risk, property, or safety decisions.

Dam height

Freeboard:

The automatic freeboard is the larger of two values: the normal freeboard from the USBR fetch table, using our estimated fetch (the longest straight line over the reservoir to the dam), and the rise of the reservoir in the inflow design flood plus the minimum freeboard for the fetch.

Spillway and power plant:

Hazard class

Spillway length

The automatic length gives a head of 1 m (3.3 ft) over the spillway at the peak inflow, at most the dam crest length. We chose 1 m arbitrarily, not from a design standard.

Downtime

Show upstream basin (dashed blue line)

Comparison

Reservoir

Volume by water level

Dam

Cross-section along the dam line, looking downstream

Spillway

Hydrology

Mean monthly inflow (NHDPlus EROM)

Flow-duration curve

Power plant

Method and limits

The dam line is burned into the DEM as a wall. Water fills every cell below the water level that connects to the upstream face of the dam (4-connected).
The default dam crest is the ground elevation of the lower abutment. The water level is dam crest minus freeboard.
The automatic freeboard is the normal freeboard from the USBR fetch table (Bureau of Reclamation 2021): 4 ft for a fetch near 0, 5 ft at 1 mi, 6 ft at 2.5 mi, 8 ft at 5 mi and 10 ft at 10 mi or more, linear between these values. The fetch is measured on the coarse elevation grid, on the reservoir that the freeboard leaves. If it is larger, the freeboard is the flood term: the highest rise of the reservoir in the inflow design flood, plus the minimum freeboard of DS-13 Table 6-8 (3 ft for a fetch near 0, 4 ft at 1 mi, 5 ft at 2.5 mi, 6 ft at 5 mi, 7 ft at 10 mi or more). Without flood data, the freeboard is the normal freeboard.
Floods: the 100- and 1,000-year peak flows and 1-, 3- and 7-day mean flows come from regional regressions on USGS gage records: a Bulletin 17C fit (PeakFQ) at each gage, then a regression on drainage area, the 24-hour design rainfall, basin slope, elevation, soil, base flow and mean precipitation, in each region. These are natural floods: upstream dams are not included. The 1,000-year values extrapolate gage records that are much shorter. The hazard class sets the inflow design flood: 1,000-year (significant) or 100-year (low).
Spillway: an uncontrolled ogee crest at the normal pool (USBR Design of Small Dams, Ch. 9), with no piers and rounded abutments. The automatic length gives a head of 1 m at the peak inflow, at most the dam crest length. The flood goes through the reservoir from the normal pool (level-pool routing of a balanced hydrograph: an NRCS unit hydrograph near the peak, inside the 1-, 3- and 7-day volumes). The reservoir area above the normal pool comes from the terrain.
If the water reaches the downstream face around a dam end, or drains to ground below the dam base, the level is lowered to the spill level.
“Optimize dam” moves each dam end in or out along the line of its end segment (at most 2 km or two times the dam length), and finds the highest dam crest whose full reservoir (dam crest minus freeboard) does not spill.
3DEP is a bare-earth surface. Under existing lakes and rivers it gives the water surface, not the bed, so the volume of existing water bodies is not included.
Upstream side and inflow come from the NHDPlus flowline that crosses the dam line with the largest drainage area. Flow is prorated to the crossing point by drainage area.
Fill volumes assume an embankment with a 10 m dam crest and 3:1 / 2:1 slopes, or a concrete gravity section with a 6 m dam crest and a 0.8:1 face. Foundations are not included.
Hydropower: the turbine type, efficiency curves, design flow (the flow exceeded 30% of the time) and minimum turbine flow come from HydroGenerate (Idaho National Laboratory). The flows are the daily flows of a USGS gage on the same river (scaled by drainage area), or else the NHDPlus monthly flows. The head is the full pool − the dam base, less 10% head loss. A reservoir that holds more than 10% of the annual inflow stores water: the turbines take the design flow while storage lasts, down to the lowest pool for the turbine type (EM 1110-2-1701), with a minimum release of 10% of the mean flow.
Comparison: a value larger than an entry on a world list gets a world rank. The lists come from Wikipedia (tallest dams, reservoirs by area, reservoirs by volume, largest dams, hydroelectric power stations, and the dam articles), the USACE National Inventory of Dams, and operator and engineering sources for concrete volumes. The lists include only built dams that impound a river: no natural dams, no tailings dams, and no dams that only raise the level of a natural water body. The lists are not complete, so the real rank can be a larger number. Hydropower is compared with mean output (annual generation ÷ 8,766 h). Other values get a size class from the percentile among US dams in the National Inventory of Dams: large (top 1%), medium (top 10%), small (top 50%), or tiny. For dam crest length the classes are very long, long, medium, and short. Dam crest length includes the dikes where the source counts them. The hydropower classes are large (100 MW or more), medium (10 MW), small (1 MW), and tiny.

Citations

U.S. Geological Survey. 3D Elevation Program (3DEP), 1/3 arc-second DEM: the terrain for the reservoir and the dam.
U.S. Environmental Protection Agency and U.S. Geological Survey. NHDPlus Version 2.1: flowlines, catchments, network connections and EROM mean flows. The upstream basins and upstream flowlines come from the catchments and the network connections.
Bureau of Reclamation (2021). Design Standards No. 13: Embankment Dams, Chapter 6: Freeboard, Table B-1 and Table 6-8: the normal and the minimum freeboard from the fetch.
Bureau of Reclamation (1987). Design of Small Dams, 3rd ed., Chapter 9: the ogee spillway equations and coefficients.
U.S. Geological Survey. Water Data API: annual peak flows and daily flows of the stream gages. GageStats: the published 100-year peaks and the regulation flags of the gages. Internet of Water, geoconnex reference gages: the NHDPlus flowline of each gage.
U.S. Geological Survey. PeakFQ 8.2.0 (Bulletin 17C flood-frequency analysis), and its regional skew map; Parrett, C., Veilleux, A., Stedinger, J.R., Barth, N.A., Knifong, D.L., and Ferris, J.C. (2011), SIR 2010-5260: the regional skew in California.
Wieczorek, M.E., Jackson, S.E., and Schwarz, G.E. (2018). Select attributes for NHDPlus version 2.1 reach catchments and modified network routed upstream watersheds, USGS data release: the basin attributes of the flood regressions.
NOAA Atlas 14; Washington State Department of Ecology, Technical Note 3 (Schaefer, Dam Safety Guidelines); Oregon Department of Transportation, SPR 656: the 100- and 1,000-year 24-hour rainfall.
U.S. Army Corps of Engineers (1993). EM 1110-2-1415, Hydrologic Frequency Analysis: the flood volumes and the balanced hydrograph. (1985). EM 1110-2-1701, Hydropower, Table 5-1: the lowest pool for each turbine type. USDA NRCS, National Engineering Handbook Part 630, Chapter 16: the dimensionless unit hydrograph.
Idaho National Laboratory. HydroGenerate (BSD 3-Clause license): turbine selection and efficiency. Tennant, D.L. (1976), Instream flow regimens for fish, wildlife, recreation and related environmental resources, Fisheries 1(4): the minimum release.
Petheram, C. and McMahon, T. A. (2019). Dams, dam costs and damnable cost overruns. Journal of Hydrology X 3, 100026. Source of the dam cost data.
U.S. Army Corps of Engineers (2026). National Structure Inventory (NSI) Base Data, 2026 release: the buildings, their residents and their values.
Nolte, C. (2020). High-resolution land value maps reveal underestimation of conservation costs in the United States. PNAS 117(47), 29577–29583. Source of the land values. Data: Dryad (CC0).
U.S. Bureau of Labor Statistics. Consumer Price Index for All Urban Consumers (CPI-U), US city average, all items: the conversion of the dollar values to one price year.
U.S. Census Bureau. Cartographic Boundary Files, 2025 states, 1:500,000: the state outlines, to name the states without population data.
U.S. Army Corps of Engineers. National Inventory of Dams: the US size classes in the comparison.
Wikipedia. Lists of tallest dams, reservoirs by surface area, reservoirs by volume, largest dams, conventional hydroelectric power stations: the world ranks in the comparison.
Nominatim: the place search.

For fun only. The results are rough estimates from public data. Do not use them for engineering, flood risk, property, or safety decisions. Provided as is, without warranty, under the Apache License 2.0.