Floodplain inundation is believed to be the dominant physical driver of an array of ecosystem patterns and processes in the Upper Mississippi River System (UMRS). A geospatial model of floodplain inundation described in Van Appledorn et al. (2021; doi: 10.1002/rra.3628) generates depth time series data for the UMRS floodplain extent. These depth time series data are typically generated from daily water surface elevations from three gaging locations along the mainstem of the Upper Mississippi River in Pool 26. However, due to missing data at the mid-pool gage location at Dixon Landing, this gage was not included in the analysis for 2009 - 2020. To understand the effect of excluding data from the mid-pool gage on modeled depth time series data, we generated depth time series data from 1990 - 1994 both with and without the gage, and compared the differences in simulated depths at select locations in Pool 26. The p26_depthdiff.csv file comprises the observed differences in depth under the two scenarios. It can be used to apply a post-hoc correction term to any depth time series data generated without the mid-pool gage, to understand the UMRS Floodplain Inundation model's sensitivity to mid-pool surface water elevation fluctuations, and other uses.