This dataset presents observations and model projections of climate, riparian cottonwood and willow seed release phenology, snowmelt streamflow timing, and relative timing of seed release and snowmelt peak streamflow in the upper South Platte River Basin, USA. We collected field observations of seed release phenology from three riparian Salicaceae species at 14 riparian sites and used those observations to develop 2-3 temperature-driven phenology models for each species. We used those phenology models, together with PRMS hydrological models of streamflow at six of the sites, to simulate historical phenology, snowmelt streamflow timing, and relative timing based on gridded temperature data from 1961-1990 and to project future phenology, snowmelt streamflow timing, and relative timing until 2098 based on downscaled CMIP3 future climate projections. The first spreadsheet in the dataset (weekly_phenology_measurements_2010-2011.csv) presents weekly observations of seed release phenology by 89 Populus deltoides spp. monilifera - plains cottonwood trees, 91 Salix amygdaloides - peachleaf willow trees, and 135 Salix exigua - narrowleaf willow shrubs in 2010 and 2011 at 14 riparian sites in the upper North and South Platte River Basins. A second spreadsheet (annual_phenology_thresholds_2010-2011.csv) presents annual dates of estimated 20% seed release by each individual at each site, which were used to calibrate phenology models. A third spreadsheet (daily_site_temperatures_2009-2011.csv) presents daily mean, maximum, and minimum temperatures at the 14 sites prior to and during phenology observations (Sept 2009 to Dec 2011; derived from Daymet gridded climate data), which also were used to calibrate phenology models. A fourth spreadsheet (annual_snowmelt-peak-timing_2010-2011.csv) presents annual dates of 50% of annual streamflow (a proxy for snowmelt peak streamflow timing) at seven sites with USGS NWIS streamflow gage data during the two years of phenology observations, for use in evaluating current relative timing of snowmelt streamflow and seed release. A fifth spreadsheet (simulated_phenology_and_snowmelt-peak-timing_1961-1990.csv) presents annual dates of 50% of annual streamflow in years with USGS NWIS streamflow gage data between 1961-1990, together with simulated annual dates of four different metrics of snowmelt peak streamflow timing and 20% seed release from each species phenology model from 1961-1990 (models forced with gridded climate data) at the six PRMS sites, for use in evaluating historic relative timing of snowmelt streamflow and seed release. A sixth dataset (projections_1962-2098.csv) presents annual and seasonal mean temperature and precipitation at the six PRMS sites and across the upstream watersheds of the six PRMS sites in 1962-2098, derived from downscaled CMIP3 future climate projections, for use in evaluating temporal trends in projected future climate at and upstream of the sites. In addition, the sixth dataset presents projected annual dates of four metrics of snowmelt peak streamflow timing and 20% seed release from each species phenology model from 1962-2098 (models forced with downscaled CMIP3 future climate projections) at the six PRMS sites, for use in evaluating temporal trends in projected future snowmelt peak streamflow timing and seed release phenology at the sites. Further, the sixth dataset presents projected numbers of days between snowmelt peak streamflow and 20% seed release for each streamflow metric and phenology model from 1962-2098 at the six PRMS sites, for use in evaluating temporal trends in projected future relative timing of snowmelt peak streamflow and seed release. A seventh spreadsheet (baseline-adjusted_projections_2020-2098) presents the same variables as the sixth spreadsheet, but adjusted to remove differences in simulated baseline conditions by subtracting simulated 1962-1990 site means from 2020-2098 annual values.