This dataset contains groundwater (GW)-altitude (ALT) data from wells that was used or considered to create a potentiometric-surface map for the Mississippi River Valley alluvial (MRVA) aquifer for spring 2018. The groundwater-altitude data was referenced to the North American Vertical Datum of 1988 (NAVD 88). Most of the wells were measured annually, but some wells were measured more than one time in a year and a small number of wells were measured continuously. Groundwater-altitude data were from wells measured in spring 2018. Spring-time measurements were preferred because water levels had generally recovered from pumping during the previous irrigation season and it was before pumping began for the current irrigation season. To best reflect hydrologic conditions in the MRVA aquifer, the groundwater altitudes used to create the 2018 potentiometric surface would be measured in a short-time frame of days or a week and there would be available data (for example from sets of wells with short-screen (about 5 to 10 feet or 1.5 to 3 meters) installed near the top, in the middle, and near the bottom of the aquifer) to indicate vertical flow components. However, most wells screened in the MRVA aquifer were measured before the potentiometric-surface map of the MRVA aquifer was planned and therefore the timing of each well’s measurement(s) was determined by the needs and schedules of the entities doing the measurements. Also, many of the measured wells had longer screens (from greater than 10 feet or 3 meters and covering a substantial part of the aquifer thickness), therefore their water-level measurements represent an average head in the aquifer for that location. The resultant potentiometric-surface contours and raster represents the generalized central tendency for spring 2018, but it would not be useful for some purposes, such as for calibration of a groundwater-flow model for early April 2018 or for some local scale assessments.