Coverage, Resolution, and Representation of Public Interests by the USGS Streamflow Monitoring Network

The US Geological Survey operates a streamflow monitoring network that extends across the United States and its territories. The network provides streamflow information to serve diverse public interests and was analyzed systematically by Konrad and others (2022) for its coverage, resolution, and representation of variables related to those interests. The variables in the network analysis include streamflow, material loads transported by streamflow, physical and human factors that affect streamflow and its quality, and the administration and management of land and water resources. The variables are publicly available, cover the contiguous United States (CONUS) or the entire Nation, and were selected to span a wide range of public interests in streamflow information. Priority areas to maintain or improve coverage, resolution, and representation of the network are identified for each variable to support network planning and operation. Priority areas for multiple variables can be overlayed to identify where different interests in monitoring are aligned. The analysis is not prescriptive in terms of the relative importance of monitoring different variables, the number of sites needed for the network, the ranking of priority areas for monitoring, or the location of monitoring sites within priority areas.

Spatial Framework

The monitoring network is 8,113 sites where USGS operated a gage that measured streamflow for at least 182 days in water year 2020. The network was simplified to no more than one site for any National Hydrography Dataset (NHD) flowline (Schwarz and Wieczorek, 2018) in CONUS or for any 12-digit hydrologic unit code (HUC12) watershed outside of CONUS (USGS 2022). In cases where there was more than one site on a flowline in CONUS or in a HUC12 outside of CONUS, the site farthest downstream was selected for the network.

Incremental gaged areas (or IGAs) are the areas between sites (the drainage area of a site that is downstream of other sites) or the entire drainage area of site without upstream sites. IGAs account for the nested structure of the monitoring network where a site may be located in the drainage area of another site. IGAs were delineated using NHD catchments for areas in the contiguous United States (CONUS) and HUC12s for areas outside of CONUS and generally extend past the gage at the downstream boundary of the IGA. The boundaries of IGAs are only approximate because of the spatial resolution of NHD catchments and HUC12s.

HUC12 watersheds are used to assess how well the network represents the spatial distribution of variables. HUC12s range in size but have a median area of 90 km2 for the United States.

Steps in the Network Analysis

The values of variables were calculated for IGAs and, if available, HUC12s. The cumulative distributions of each variable were compiled for the United States and for each major river basin (4-digit hydrologic unit code) and used to calculate network metrics and to identify priority areas for maintaining or adding monitoring sites.

Network Metrics

Three metrics are used to evaluate a monitoring network for each variable: coverage, resolution, and representation. The metrics are calculated from the cumulative distributions of the variables, shown below. Each metric varies from 0 to 1. Metrics are not displayed for multi-category variables (Climate Divisions, Ecoregions, Surficial Geology Types).

Coverage is the fraction of the total value of a variable that is found in areas that are monitored: 0 - none of the variable is in monitored areas; 1 - all of the variable is in monitored areas. Coverage can only be calculate for variables with known values outside of the monitoring network. .

Resolution indicates the deviation of the cumualtive distribution of IGA values from the mean IGA value: 0 - the value of a variable for monitored areas is concentrated entirely in one IGA, 1 - all IGAs have equal value (the mean IGA value of the variable). Resolution can only be calculated for spatially-conserved variables (count, length, area, volume, mass, or fluxes). The resolution of characteristics such as elevation, drainage area, temperature, or fraction of stream length or area with a designation or classification is based on either stream length or land surface area with the characteristic.

Representation indicates whether IGA values are evenly distributed among the decile intervals (0-10%, 10%-20%,...) of a benchmark distribution: 0 - all IGA values are outside the range of the benchmark distribution, 1 - 10% of IGAs are in each of the decile intervals of the benchmark distribution. Typically, HUC12s values are used for the benchmark distribution. If the HUC12 values are unknown, 10% intervals of the IGA range are used to calculate representation: ~0 - nearly all of the total value of a variable is in one interval of the range of values for IGAs, 1 - IGA values are linearly distributed (10% of IGAs are in each of intervals of the range of IGA values). Representation can be calculated for characteristics such as mean elevation or a percent of area in a land cover category, but they must be associated with a spatially-conserved variable such as stream length or land suraface area to create the distributions.

Network objectives can vary for different variables (for example, resolution of streamflow and representation of basin elevation) and should be considered when interpreting these metrics.

Priority Designations of IGAs and HUC12s

IGAs that are important for network coverage, resolution, or representations are designated as priorities to maintain monitoring sites. IGAs and HUC12 where additional gages would substantially improve network coverage, resolution, or representation are designated as priorities to add monitoring sites. IGAs are defined by upstream and downstream sites, so a site is a monitoring priority if it is at the upstream or downstream boundary of a priority IGA. The number of priorities IGAs is not a recommendation about the number of sites needed for monitoring. Indeed, a variable with few priority IGA simply indicates that there are a few areas of particularly high importantce for monitoring and that it may be possible to re-configure - but not eliminate - sites in non-priority IGAs without impacting network performance.

Priority designations are determined by applying consistent rules to the cumulative distributions of each variable. Generally priority areas have higher values of a variable but they may also be part of a variable's distribution that is not well represented by the monitoring network. The cumulative distributions shown for each variable include the specific criteria used for priority designations. Streamflow and material loads are not available for HUC12s, so HUC12s do not have priority designations for these variables. Ungaged areas (shown in the HUC12 layer as orange) can be considered as priorities to add gages for these variables.

Priority areas are not a network design. Network design including the total number of monitoring sites in the network and the specific locations for monitoring sites depends on the objectives and resources available for monitoring and considerations of access and safety. Priority designations are based on the USGS streamflow gaging network active in water year 2020 and could change as sites are added or removed from the network. Priority designations do not reflect legal requirements or local needs for streamflow information at specific locations. Any specific changes to a monitoring network should assess local information needs, access, and safety.

Viewing Variables and Monitoring Priorities

The map viewer has layers with monitoring sites, incremental gaged areas (IGAs), the priority designation of IGAs for individual variables, 12-digit hydrologic unit code watersheds (HUC12s), and the priority designations of HUC12s for individual variables.

Network Analysis Results

Download results from ScienceBase,

Variables and priority designations for IGAs and HUC12s have been aggregated into eight topics and summarized for the Nation and each major river basin.

National Results (select a topic)

The network analysis has been approved for release by the U.S. Geological Survey (USGS). Although the data have been subjected to rigorous review and is substantially complete, the USGS reserves the right to revise the data pursuant to further analysis and review. Furthermore, the data are released on condition that neither the USGS nor the U.S. Government shall be held liable for any damages resulting from its authorized or unauthorized use.


Konrad, C.P., Anderson, S.W., Restivo, D.E., and David, J.E., 2022, Network Analysis of USGS Streamflow Gages, U.S. Geological Survey Data Release, accessed on August 26, 2022 at

Schwarz, G.E., and Wieczorek, M.E., 2018, Database of modified routing for NHDPlus Version 2.1 flowlines: ENHDPlusV2, 2018: U.S. Geological Survey data release, accessed on January 10, 2022 at

US Geological Survey, 2022, Watershed Boundary Dataset, accessed on April 12, 2022 at