This dataset contains annual peak and seasonal maximum streamflow data from the USGS National Water Information System (NWIS) and the results of an analysis of spatial and temporal patterns for those data for the conterminous U.S. An interpretation of the analysis of these data will be published in a journal article. The streamflow data were compiled for the years 1966 to 2015 for 415 streamgages that are part of the USGS HCDN-2009 network. The HCDN-2009 network contains streamgages in watersheds with minimal anthropogenic change. The dataset contains the annual peak and the maximum daily streamflow for the months of October through December (OND), January through March (JFM), April through June (AMJ), and July through September (JAS). The maximum daily streamflow for OND, JFM, AMJ, and JAS was determined as the largest daily streamflow over those months. This dataset contains the following files for the annual peaks and maximum daily streamflow. The files labeled "raw" contain the water year and the raw annual peaks from NWIS and the selected maximum daily streamflow from NWIS. The files labeled "SPI" contain the water year and the standardized peaks and maximum daily streamflow by using the Standardized Precipitation Index method. The files labeled "cluster_assignments" contain (in columns) the gage name (two letter abbreviation of the state in which the gage is located and the USGS site number), the latitude and longitude (NAD 83) of the gage, an integer that is an identifier of the cluster that the gage was assigned, and the Pearson correlation coefficient between the standardized streamflow and the average standardized streamflow of all gages in its cluster. The annual peaks were clustered into 4 groups, the OND maximum flows were clustered into 7 groups, the JFM maximum flows were clustered into 9 groups, and the both the AMJ and JAS maximum flows were clustered into 5 groups. The files with the term "cluster_means" in the file name contain the water year and the mean of the standardized streamflow of all gages in the clusters of peaks and maximum daily streamflow for OND, JFM, AMJ, and JAS.