StreamStats Version 4: Streamflow Statistics for Any U.S. Stream Site

From a U.S. Geological Survey fact sheet by Kernell G. Ries III, Jeremy K. Newson, Martyn J. Smith, John D. Guthrie, Peter A. Steeves, Tana L. Haluska, Katharine R. Kolb, Ryan F. Thompson, Richard D. Santoro and Hans W. Vraga, 2017.

StreamStats version 4, at https://streamstats.usgs.gov, is a USGS map-based web application with tools for water-resources planning, management and engineering. Its main job is to provide estimates of streamflow statistics for USGS streamgages — where streamflow data are collected — and for any ungaged site a user picks on a stream.

Why streamflow statistics matter

Engineers, land managers, biologists and many others rely on statistics such as the 1-percent flood, the mean flow and the 7-day, 10-year low flow. The 1-percent flood — exceeded on average once in 100 years, with a 1-percent chance in any year — underlies floodplain maps used to set insurance rates and zoning. Streamflow statistics also inform:

• the design of dams, bridges and culverts;
• water-supply planning and management;
• permits for water withdrawals and for wastewater and industrial discharges;
• the design and regulation of hydropower plants;
• minimum flows set to protect freshwater ecosystems.

They can be computed from data at streamgages, but they are most often needed at ungaged sites, where there are no data.

What it does

StreamStats' GIS tools let users draw the drainage-basin boundary for a chosen site, compute its basin characteristics and estimate its streamflow statistics — much faster than the old manual methods. Users can also get descriptive information and published basin characteristics and statistics for USGS streamgages. Other tools edit basin boundaries and measure distances and elevation profiles, and some states have their own: tests for upstream dam regulation in Colorado and Montana, and summaries of water withdrawals and discharges in the Delaware River Basin and northeastern Ohio. Tools that navigate along the digital stream network were in development.

The home page links to News (new states and methods, problems, outages), Documentation (definitions, a users' manual and more), How it Works, Web Services, the Batch Tool and Help.

The interface

• Map Frame: most of the screen, showing base maps and data layers and letting users select sites. Buttons open exploration tools (streamgage queries, distances, elevation profiles, current location), switch layers on and off, and zoom.
• Sidebar: a series of banners that turn from grey to blue as each step activates — select a state or region (search by address, zip code or state), identify a study area (delineate, edit and download the basin), select scenarios (which statistics and basin characteristics to compute), and build a report, which can be printed, saved or exported with the basin in GIS formats.
• Help, About and Report links above the map give the users' manual and support, information about the selected state or region, and a report of the results so far.

The USGS strongly recommends reading the documentation before use — especially the state or regional information, and the reports it lists, before estimating statistics for ungaged sites.

Streamgages

StreamStats offers nearly 500,000 basin characteristics and 1.5 million streamflow statistics for more than 36,000 USGS streamgages: about 8,500 operating continuously, about 12,000 discontinued, plus partial-record stations (measured systematically over years to estimate peak or low flows) and miscellaneous-measurement stations (for particular studies). Coloured triangles on the map mark each type; clicking one shows its descriptive information, basin characteristics and statistics. More than 850 kinds of basin characteristics and more than 2,650 kinds of statistics exist across the stations, though each station usually has only a few, and every value is cited to where it was previously computed.

Ungaged sites

StreamStats uses regional regression equations, mostly published by the USGS, that relate streamflow statistics to basin characteristics at a region's streamgages. Measure a site's basin characteristics, put them into the equations, and you have estimates. The USGS has such equations for peak-flow statistics, such as the 100-year flood, in every state, and for other statistics in many states. Vermont's equation for the 100-year flood, for example, is:

PK100 = 0.251 × DA^0.854 × (S + 1)^−0.297 × P^1.089

where PK100 is the peak flow expected on average once in 100 years (a 1-percent chance in any year), in cubic feet per second; DA is the drainage area in square miles; S is the percentage of the basin covered by wetlands and water; and P is the mean annual precipitation in inches.

The Sidebar automates the whole process — locate the site, delineate the basin, compute characteristics, estimate the statistics and produce a report with a map — saving from an hour to a few days of work depending on the basin. Basins are delineated from a Digital Elevation Model (DEM), a grid of land-surface elevations, together with the digital stream network; for most states the DEM has been adjusted to match the stream network and existing drainage boundaries, which usually makes delineations more accurate. A version 2 tool that estimated statistics from flow per unit area at a nearby gage was being rebuilt for version 4.

Limitations

• Check the results: the underlying datasets are large and complex, often made by others, and can contain errors — basin delineations especially. The site has tools and base maps for checking and editing them.
• Natural flow is assumed, unless the report behind the equations says otherwise. Where dams or withdrawals substantially change the timing, size or duration of flows, users must adjust the estimates.
• Gage and equation estimates differ: both carry errors, so some disagreement is expected, and where flows are affected by human activity the difference should not be read as that activity's effect.
• Extrapolation: outputs include error indicators and the range of each basin characteristic among the gages used to build the equations. If a site falls outside a range, StreamStats warns that it is extrapolating, and gives no error indicator, because the errors are unknown and may be very large.

Web services and batch tool

Everything StreamStats does is available as web services, so other applications can request, say, a basin delineation over the internet without assembling their own GIS data. The batch tool processes many sites at once, returning shapefiles of basins, characteristics and statistics; users upload a shapefile of points already aligned with the StreamStats stream grid.

Sources

• Kernell G. Ries III and others, StreamStats, Version 4, U.S. Geological Survey Fact Sheet 2017–3046 (superseding Fact Sheet 2008–3067). https://doi.org/10.3133/fs20173046 — the Vermont example recovered from the PDF, where the import had kept only part of the equation.
• The Vermont equation is from Olson (2014), U.S. Geological Survey Scientific Investigations Report 2014–5078; earlier versions are described by Ries and others (2004, 2008).
• Correction: the fact sheet's "1-percent change of occurrence" is read as 1-percent chance.
• Rewritten in hubnx's own words.