This data release consists of the following components: Sex ratio data from cottonwood trees at random points on the floodplain in the North and South units of Theodore Roosevelt National Park, ND. These data were used to investigate the effects of age, height above, and distance from the channel on mortality of male and female trees of plains cottonwood (Populus deltoides subsp. monilifera) as described in the Friedman and Griffin (2017) report. Tree core and tree ring data from the North and South Units of Theodore Roosevelt National Park, North Dakota. South Unit data was collected in April 2012, North Unit data was collected in the summer and fall of 2010. The trees are located on the floodplain of the Little Missouri River in Theodore Roosevelt National Park. These data were used to reconstruct flow and climate as described in the Friedman and Griffin (2017) report and in other documents cited by that report. The tree ring data is presented in Standard Tucson format. Floodplain and riparian cottonwood forest areas in the South Unit were digitized as separate shapefiles using 2010 NAIP imagery. They were mapped to assist management of cottonwood forests by increasing understanding of the relation between geomorphic setting, flow, precipitation, temperature, and other factors. Edges of water, channel centerline, valley bottom centerline, extent of valley bottom, and estimated bankfull channel data for the Little Missouri River in the North and South Units were mapped as separate shapefiles from 2010 NAIP imagery as well.