Paleohydrologic records provide a valuable perspective on the variability of streamflow and hydroclimate that is critical for water resource planning and placing present day and future conditions into a long-term context. Until now, key insights gained from streamflow reconstructions in the other river basins across the Western U.S. have been lacking in the Milk and St. Mary River Basin. Here we utilize a new database of naturalized streamflow records for the Milk and St. Mary Rivers and an expanded network of tree-ring records from the region to reconstruct streamflow at eight gaging locations located in the mountains, foothills, and plains reaches of the basins. The network of streamflow reconstructions presented here were generated for use by the Bureau of Reclamation and Montana Department of Natural Resources and Conservation in the Basin Study update for the Milk and St. Mary Rivers, and provides important data resources to water managers balancing increasing water demands for hydropower, irrigation, and ecological resources with increasing drought and flood risk in the basin.

Paleohydrologic records provide a valuable perspective on the variability of streamflow and hydroclimate that is critical for water resource planning and placing present day and future conditions into a long-term context. Until now, key insights gained from streamflow reconstructions in the other river basins across the Western U.S. been lacking in the Upper Missouri River Basin due to a lack of extended streamflow records. Here we utilize a new database of naturalized streamflow records for the Upper Missouri and an expanded network of tree-ring records from the region to reconstruct streamflow at 31 gaging locations across the major Mountain Headwaters of the United States’ largest river basin. The database also includes an Upper Missouri Basin Basin composite record of streamflow that is not specific to any streamgage location, but rather summarizes streamflow variability across all the major gaging locations in the Upper Missouri River. The reconstructions explain an average of 68% of the variability in the observed streamflow records and extend records of streamflow to C.E. 886 on average. The network of streamflow reconstructions presented here fills a major geographical void in paleohydrologic understanding and provides important data resources to water managers balancing increasing water demands for hydropower, irrigation, navigation, and ecological resources with increasing flood risk in the basin.

Paleohydrologic records provide a valuable perspective on the variability of streamflow and hydroclimate that is critical for water resource planning and placing present day and future conditions into a long-term context. Until now, key insights gained from streamflow reconstructions in the other river basins across the Western U.S. been lacking in the Upper Missouri River Basin due to a lack of extended streamflow records. Here we utilize a new database of naturalized streamflow records for the Upper Missouri and an expanded network of tree-ring records from the region to reconstruct streamflow at 31 gaging locations across the major Mountain Headwaters of the United States’ largest river basin. The database also includes an Upper Missouri Basin Basin composite record of streamflow that is not specific to any streamgage location, but rather summarizes streamflow variability across all the major gaging locations in the Upper Missouri River. The reconstructions explain an average of 68% of the variability in the observed streamflow records and extend records of streamflow to C.E. 886 on average. The network of streamflow reconstructions presented here fills a major geographical void in paleohydrologic understanding and provides important data resources to water managers balancing increasing water demands for hydropower, irrigation, navigation, and ecological resources with increasing flood risk in the basin.

Tree-ring reconstructions of water-year (Oct 1 through Sep 30th) flow for 31 gaging sites in Missouri River basin, with complete data for 1685 through 1977 (n = 293 water years). The complete 105 tree-ring reconstructions of streamflow used in the Nature Scientific Reports paper were obtained from various sources; 74 flow reconstructions were obtained from the web resource, TreeFlow (http://www.treeflow.info/), and an additional 31 flow reconstructions (provided here) for the Missouri River basin were obtained from the U.S. Geological Survey in Bozeman, Montana.

Tree-ring reconstructions of water-year (Oct 1 through Sep 30th) flow for 31 gaging sites in Missouri River basin, with complete data for 1685 through 1977 (n = 293 water years). The complete 105 tree-ring reconstructions of streamflow used in the Nature Scientific Reports paper were obtained from various sources; 74 flow reconstructions were obtained from the web resource, TreeFlow (http://www.treeflow.info/), and an additional 31 flow reconstructions (provided here) for the Missouri River basin were obtained from the U.S. Geological Survey in Bozeman, Montana.

This data release contains trend results computed on the basis of modeled and observed daily streamflows at 502 reference gages across the conterminous U.S. from October 1, 1983 through September 30, 2016. Modeled daily streamflows were computed using the deterministic Precipitation Runoff Modeling System (PRMS), and five statistical techniques: Nearest-Neighbor Drainage Area Ratio (NNDAR), Map-Correlation Drainage Area Ratio (MCDAR), Ordinary Kriging of the logarithms of discharge per unit area (OKDAR), Nearest-Neighbor nonlinear spatial interpolation using flow duration curves (NNQPPQ), and Map-Correlation nonlinear spatial interpolation using flow duration curves (MCQPPQ). Observed daily streamflow data for the study gages were retrieved from the National Water Information System (NWIS). Study gages were selected from among Hydro-Climatic Data Network 2009 (HCDN-2009) gages in the GAGES-II dataset considered to be minimally affected by regulation, diversion, mining, or other anthropogenic activities. Results include trends in annual and monthly means, annual percentiles (1, 5, 10, 25, 50, 75, 90, 95, 99), annual 1-day high, 3-day high, and 7-day low, and annual snowmelt-related runoff timing for a subset of snowmelt dominated basins. Bias and volumetric efficiency statistics between observed and modeled streamflows also are provided.

This data release contains trend results computed on the basis of modeled and observed daily streamflows at 502 reference gages across the conterminous U.S. from October 1, 1983 through September 30, 2016. Modeled daily streamflows were computed using the deterministic Precipitation Runoff Modeling System (PRMS), and five statistical techniques: Nearest-Neighbor Drainage Area Ratio (NNDAR), Map-Correlation Drainage Area Ratio (MCDAR), Ordinary Kriging of the logarithms of discharge per unit area (OKDAR), Nearest-Neighbor nonlinear spatial interpolation using flow duration curves (NNQPPQ), and Map-Correlation nonlinear spatial interpolation using flow duration curves (MCQPPQ). Observed daily streamflow data for the study gages were retrieved from the National Water Information System (NWIS). Study gages were selected from among Hydro-Climatic Data Network 2009 (HCDN-2009) gages in the GAGES-II dataset considered to be minimally affected by regulation, diversion, mining, or other anthropogenic activities. Results include trends in annual and monthly means, annual percentiles (1, 5, 10, 25, 50, 75, 90, 95, 99), annual 1-day high, 3-day high, and 7-day low, and annual snowmelt-related runoff timing for a subset of snowmelt dominated basins. Bias and volumetric efficiency statistics between observed and modeled streamflows also are provided.

This data release contains daily time series estimates of natural streamflow for 1,385 streamgages in 19 study regions in the conterminous U.S. from October 1, 1980, through September 30, 2017. These estimates are provided for gages from mostly undisturbed watersheds as defined by Falcone (2011), using five statistical techniques: nearest-neighbor drainage area ratio (NNDAR), map-correlation drainage area ratio (MCDAR), nearest-neighbor nonlinear spatial interpolation using flow duration curves (NNQPPQ), map-correlation nonlinear spatial interpolation using flow duration curves (MCQPPQ), and ordinary kriging of the logarithms of discharge per unit area (OKDAR). Location information and basin characteristics for study gages were obtained from the "Reference" gages of the GAGES-II dataset (Falcone, 2011, https://water.usgs.gov/lookup/getspatial?gagesII_Sept2011). Observed daily streamflow data were retrieved from the National Water Information System (NWIS) on September 7, 2018. NNDAR, MCDAR, NNQPPQ, and MCQPPQ estimates were computed following methods described by Farmer and others (2014), with updates to the flow-duration curve modeling which is described by Over and others (2018). OKDAR estimates were computed using pooled variograms for each study region following methods described by Farmer (2016). Daily streamflow estimation was conducted in a leave-one-out-cross-validation approach where each streamgage was treated as if ungaged and all the remaining streamgages in a study region were used to calibrate each method and perform estimations at the "ungaged" site. The observed streamflow records were compared to the five simulated streamflow records to help assess performance of each method. These performance metrics are provided at each gage for all five statistical methods. References cited: Falcone, J.A., 2011, GAGES-II: Geospatial Attributes of Gages for Evaluating Streamflow [digital spatial dataset] : U.S. Geological Survey Water Resources NSDI Node web page, https://water.usgs.gov/lookup/getspatial?gagesII_Sept2011. Farmer, W.H., Archfield, S.A., Over, T.M., Hay, L.E., LaFontaine, J.H., and Kiang, J.E., 2014, A comparison of methods to predict historical daily streamflow time series in the southeastern United States: U.S. Geological Survey Scientific Investigations Report 2014–5231, 34 p., http://dx.doi.org/10.3133/sir20145231. Farmer, W. H., 2016, Ordinary kriging as a tool to estimate historical daily streamflow records, Hydrology and Earth System Sciences, 20, 2721-2735, https://doi.org/10.5194/hess-20-2721-2016. Over, T.M., Farmer, W.H., Russell, A.M., 2018, Refinement of a regression-based method for prediction of flow-duration curves of daily streamflow in the conterminous United States; U.S. Geological Survey Scientific Investigations Report 2018–5072, https://doi.org/10.3133/sir20185072.

This data release contains daily time series estimates of natural streamflow for 1,385 streamgages in 19 study regions in the conterminous U.S. from October 1, 1980, through September 30, 2017. These estimates are provided for gages from mostly undisturbed watersheds as defined by Falcone (2011), using five statistical techniques: nearest-neighbor drainage area ratio (NNDAR), map-correlation drainage area ratio (MCDAR), nearest-neighbor nonlinear spatial interpolation using flow duration curves (NNQPPQ), map-correlation nonlinear spatial interpolation using flow duration curves (MCQPPQ), and ordinary kriging of the logarithms of discharge per unit area (OKDAR). Location information and basin characteristics for study gages were obtained from the "Reference" gages of the GAGES-II dataset (Falcone, 2011, https://water.usgs.gov/lookup/getspatial?gagesII_Sept2011). Observed daily streamflow data were retrieved from the National Water Information System (NWIS) on September 7, 2018. NNDAR, MCDAR, NNQPPQ, and MCQPPQ estimates were computed following methods described by Farmer and others (2014), with updates to the flow-duration curve modeling which is described by Over and others (2018). OKDAR estimates were computed using pooled variograms for each study region following methods described by Farmer (2016). Daily streamflow estimation was conducted in a leave-one-out-cross-validation approach where each streamgage was treated as if ungaged and all the remaining streamgages in a study region were used to calibrate each method and perform estimations at the "ungaged" site. The observed streamflow records were compared to the five simulated streamflow records to help assess performance of each method. These performance metrics are provided at each gage for all five statistical methods. References cited: Falcone, J.A., 2011, GAGES-II: Geospatial Attributes of Gages for Evaluating Streamflow [digital spatial dataset] : U.S. Geological Survey Water Resources NSDI Node web page, https://water.usgs.gov/lookup/getspatial?gagesII_Sept2011. Farmer, W.H., Archfield, S.A., Over, T.M., Hay, L.E., LaFontaine, J.H., and Kiang, J.E., 2014, A comparison of methods to predict historical daily streamflow time series in the southeastern United States: U.S. Geological Survey Scientific Investigations Report 2014–5231, 34 p., http://dx.doi.org/10.3133/sir20145231. Farmer, W. H., 2016, Ordinary kriging as a tool to estimate historical daily streamflow records, Hydrology and Earth System Sciences, 20, 2721-2735, https://doi.org/10.5194/hess-20-2721-2016. Over, T.M., Farmer, W.H., Russell, A.M., 2018, Refinement of a regression-based method for prediction of flow-duration curves of daily streamflow in the conterminous United States; U.S. Geological Survey Scientific Investigations Report 2018–5072, https://doi.org/10.3133/sir20185072.

This data release includes datasets with annual streamflow statistics determined with the Indicators of Hydrologic Alteration software and R source code to create time-series plots with overlaid locally weighted scatterplot smoothing (lowess) lines.