A major rain event caused catastrophic flooding from July 9 through 12, 2023 in various portions of the State of Vermont, resulting in millions of dollars of damage. This is one of 3 datasets in a data release that documents the flood data and analyses. This part of the data release contains the peak streamflow and lake level data resulting from the July 9 through 12, 2023 flooding as well as the flood-frequency analysis that was completed implementing the Bulletin 17C guidelines (https://doi.org/10.3133/tm4B5). Flood flows were estimated for the 50-, 20-, 10-, 4-, 2-, 1-, 0.5-, and 0.2-percent annual exceedance probabilities (AEPs) for all streamgages operated in Vermont by the USGS where data was collected. This data release presents frequency analyses for 80 streamgages and 2 lake gages in Vermont and an additional 6 sites where peak streamflows were determined using indirect measurement techniques. The data release contains the input and output files for the version 7.5.1 PeakFQ software (https://water.usgs.gov/software/PeakFQ/; Veilleux and others, 2014) used to conduct flood-frequency analyses using the expected moments algorithm (England and others, 2018). The peak-flow files used as input to PeakFQ were obtained from the USGS National Water Information System (NWIS) database (https://nwis.waterdata.usgs.gov/usa/nwis/peak) and incorporate annual peak flows through water year 2023. Results of the flood-frequency analyses at streamgages that had unregulated peak streamflows were weighted with regional flood-frequency regression equation results (Olson, 2014). The flood frequency for the two lake gages was estimated using HEC-SSP version 2.2 (https://www.hec.usace.army.mil/software/hec-ssp/) because the software allows flood frequency analysis without logarithmic transformation of the data. In addition to the input and output for the PeakFQ software, this data release has four Excel tables summarizing the flood data, the flood frequency results, and a summary table of information describing the data and analysis for each streamgage.

A major rain event caused catastrophic flooding from July 9 through 12, 2023 in various portions of the State of Vermont, resulting in millions of dollars of damage. These data are one of three datasets in the Vermont Flood of July 2023 Data data release. This part of the data release contains high-water mark (HWM) data from the Vermont flood of July 2023. The Comma-Separated Values (CSV) file table_JulyHWMs.csv contains 547 surveyed HWMs on 31 rivers from the Vermont flood of July 2023. In addition, a comparison between surveyed HWM data from the July 2023 flood and from Tropical Storm Irene in 2011 is included in the data release. The data from the two events--where HWMs were co-located and surveyed--are compared in the file table_HWMcompare.csv. Elevation differences were calculated as the July 2023 HWM surveyed elevation minus Tropical Storm Irene HWM surveyed elevation at the same site. Sites are defined as areas on a river that are expected to have the same hydraulic setting, for example, immediately downstream of a bridge. These data are also hosted on the USGS Flood Event Viewer (https://stn.wim.usgs.gov/FEV/).

A major rain event caused catastrophic flooding from July 9 through 12, 2023 in various portions of the State of Vermont, resulting in millions of dollars of damage. These data are one of three datasets in the Vermont Flood of July 2023 Data data release. This part of the data release contains high-water mark (HWM) data from the Vermont flood of July 2023. The Comma-Separated Values (CSV) file table_JulyHWMs.csv contains 547 surveyed HWMs on 31 rivers from the Vermont flood of July 2023. In addition, a comparison between surveyed HWM data from the July 2023 flood and from Tropical Storm Irene in 2011 is included in the data release. The data from the two events--where HWMs were co-located and surveyed--are compared in the file table_HWMcompare.csv. Elevation differences were calculated as the July 2023 HWM surveyed elevation minus Tropical Storm Irene HWM surveyed elevation at the same site. Sites are defined as areas on a river that are expected to have the same hydraulic setting, for example, immediately downstream of a bridge. These data are also hosted on the USGS Flood Event Viewer (https://stn.wim.usgs.gov/FEV/).

These data include annual instantaneous maximum discharge record for 156 streams and rivers in Vermont and adjacent areas of New Hampshire, Massachusetts, and New York through the 2023 water year. The data also include flood-frequency estimates for the 50-, 20-, 10-, 4-, 2-, 1-, 0.5-, and 0.2-percent annual exceedance probabilities that were analyzed using the discharge data. These flood-frequency data were used to develop regression equations for estimating the magnitude of floods at the selected annual exceedance probabilities on ungaged, unregulated, rural streams in Vermont. The regression equations use the basin characteristics of drainage area, percentage of wetland area in the basin, and the basinwide mean of the annual precipitation as explanatory variables. These basin characteristics for the streamgages and the Geographic Information System datasets for computing the basin characteristics are also in this data release. This data release is structured with the streamgage data, the input and output files for the version 7.5.1 PeakFQ software (U.S. Geological Survey, 2024) used to conduct flood-frequency analysis, the flood-frequency results, the streamgage basin characteristics, and the geographic information system datasets used to determine the basin characteristics on the main page. There is one sub-page which is used as a model archive for the regression analysis. U.S. Geological Survey, 2024, Water resources application software, PeakFQ, accessed March 4, 2024, at http://water.usgs.gov/software/peakfq.html.

These data include flood-frequency data for the 50-, 20-, 10-, 4-, 2-, 1-, 0.5-, and 0.2-percent annual exceedance probabilities (pfqprocessed.zip) and the basin characteristics (siteinfoVT.txt) for 156 streamgages in Vermont and adjacent areas in surrounding states. These data were used as input to the regression model developed for estimating the magnitude of floods at the selected annual exceedance probabilities on ungaged, unregulated, rural streams in Vermont. The model code is provided in R script language (R Core Team, 2024) and utilized the generalized-least-squares regression tools in the WREG package (Farmer, 2021) to develop the regression equations to estimated flood discharge in Vermont. The model output (VT_Regression_Equations.pdf) provides code and the regression equations. Farmer, W.H., 2021, WREG—Weighted least squares regression for streamflow frequency statistics (ver. 3.0): U.S. Geological Survey software release, accessed January 31, 2025, at https://doi.org/10.5066/P9ZCGLI1. R Core Team, 2024, R: A language and environment for statistical computing version 4.4.0: R Foundation for Statistical Computing, Vienna, Austria, accessed July 18, 2024 at, https://www.R-project.org.

Flood Insurance Studies (FISs) published by the Federal Emergency Management Agency (FEMA) provide 10-, 2-, 1-, and 0.2-percent annual exceedance probability (AEP) streamflows and water-surface elevations for many stream reaches that were affected by the July 2023 flood in Vermont. After a flood, it is useful to compare observed streamflows and water-surface elevations to flood flow-frequency and water-surface elevations published in a FIS report (Federal Emergency Management Agency, 1980–2014). Theoretically, the AEP for the peak water-surface elevation and the AEP for the peak streamflow from the same event at the same location should be the same. Reaches for which the AEPs do not align show where existing FISs could be updated with new hydrologic data and hydraulic models. Flood streamflows for 10-, 2-, 1- and 0.2-percent frequencies using annual peak-flow data through water year 2023 were compared to peak streamflows in FEMA FIS reports and pending (2024) FIS studies. The proposed peak streamflows from pending FIS studies are based on statistical analyses of annual peak flow data prior to water year 2023. Although flood-frequency estimates were updated for 80 streamgages after the July 2023 flood, only a fraction of the streamgages have detailed hydrologic analysis in FIS reports (or pending FIS reports) and can be used in the streamflow comparison. Peak-water surface elevations from the July 2023 flood were also documented at surveyed high-water marks (HWMs) along selected river reaches impacted by the July 2023 flood in Vermont. Reaches that had HWMs, a streamgage on the same river with a drainage area within fifty percent of the reach drainage area, and an effective FIS were selected for evaluation. Streamflows and HWMs from the July flood event are presented alongside statistical streamflows and elevations for the 10- to 0.2-percent AEPs from the applicable FIS.

Flood Insurance Studies (FISs) published by the Federal Emergency Management Agency (FEMA) provide 10-, 2-, 1-, and 0.2-percent annual exceedance probability (AEP) streamflows and water-surface elevations for many stream reaches that were affected by the July 2023 flood in Vermont. After a flood, it is useful to compare observed streamflows and water-surface elevations to flood flow-frequency and water-surface elevations published in a FIS report (Federal Emergency Management Agency, 1980–2014). Theoretically, the AEP for the peak water-surface elevation and the AEP for the peak streamflow from the same event at the same location should be the same. Reaches for which the AEPs do not align show where existing FISs could be updated with new hydrologic data and hydraulic models. Flood streamflows for 10-, 2-, 1- and 0.2-percent frequencies using annual peak-flow data through water year 2023 were compared to peak streamflows in FEMA FIS reports and pending (2024) FIS studies. The proposed peak streamflows from pending FIS studies are based on statistical analyses of annual peak flow data prior to water year 2023. Although flood-frequency estimates were updated for 80 streamgages after the July 2023 flood, only a fraction of the streamgages have detailed hydrologic analysis in FIS reports (or pending FIS reports) and can be used in the streamflow comparison. Peak-water surface elevations from the July 2023 flood were also documented at surveyed high-water marks (HWMs) along selected river reaches impacted by the July 2023 flood in Vermont. Reaches that had HWMs, a streamgage on the same river with a drainage area within fifty percent of the reach drainage area, and an effective FIS were selected for evaluation. Streamflows and HWMs from the July flood event are presented alongside statistical streamflows and elevations for the 10- to 0.2-percent AEPs from the applicable FIS.

The U.S. Geological Survey (USGS), in cooperation with Connecticut Department of Transportation, completed a study to improve flood-frequency estimates in Connecticut. This companion data release is a Microsoft Excel workbook for: (1) computing flood discharges for the 50- to 0.2-percent annual exceedance probabilities from peak-flow regression equations, and (2) computing additional prediction intervals, not available through the USGS StreamStats web application. The current StreamStats application (version 4) only computes the 90-percent prediction interval for stream sites in Connecticut. The Excel workbook can be used to compute the 70-, 80-, 90-, 95-, and 99-percent prediction intervals. The prediction interval provides upper and lower limits of the estimated flood discharge with a certain probability, or level of confidence in the accuracy of the estimate. The standard error of prediction for the Connecticut peak-flow regression equations ranged from 26.3 to 45.0 percent (Ahearn and Hodgkins, 2020). The Excel workbook consists of four worksheets. The worksheets provide an overview of how the application works; input and output tables of the explanatory variables and flood discharges, and graphical display of the results; and the computational formulas used to estimate the flood discharges and prediction intervals.

The U.S. Geological Survey (USGS), in cooperation with Connecticut Department of Transportation, completed a study to improve flood-frequency estimates in Connecticut. This companion data release is a Microsoft Excel workbook for: (1) computing flood discharges for the 50- to 0.2-percent annual exceedance probabilities from peak-flow regression equations, and (2) computing additional prediction intervals, not available through the USGS StreamStats web application. The current StreamStats application (version 4) only computes the 90-percent prediction interval for stream sites in Connecticut. The Excel workbook can be used to compute the 70-, 80-, 90-, 95-, and 99-percent prediction intervals. The prediction interval provides upper and lower limits of the estimated flood discharge with a certain probability, or level of confidence in the accuracy of the estimate. The standard error of prediction for the Connecticut peak-flow regression equations ranged from 26.3 to 45.0 percent (Ahearn and Hodgkins, 2020). The Excel workbook consists of four worksheets. The worksheets provide an overview of how the application works; input and output tables of the explanatory variables and flood discharges, and graphical display of the results; and the computational formulas used to estimate the flood discharges and prediction intervals.

Current estimates of the magnitude and frequency of floods at gaged and ungaged stream sites are critical for assessing flood risk, delineating flood zones, designing hydraulic structures, and managing flood plains. The Connecticut Department of Transportation collaborated with U.S. Geological Survey (USGS) in a study to improve the flood-frequency estimates in Connecticut and develop regional regression equations for estimating annual exceedance probability discharges at ungaged sites in Connecticut. The results of the study are found in Scientific Investigations Report (add link). This companion data release consists of data compiled and used for the flood-frequency analysis of annual peak flows and development of regional regression equations for Connecticut. The data release is composed of four tables that include: 1) descriptions and locations of streamgages for flood frequency analysis of annual peak flows; 2) updated flood discharges for 152 streamgages for the 50-, 20-, 10-, 4-, 2-, 1-, 0.5-, and 0.2-percent annual exceedance probabilities (AEP) using annual peak data through water year 2015; 3) representation of peak flow data using perception thresholds and interval data in the flood frequency analysis to describe uncertainty and represent missing and historical records; and 4) basin and climatic characteristics that were included as explanatory variables in the regression equations.