Expansion of aquatic and marsh area into once forest and agricultural area reflects changing hydrological conditions along the Upper Mississippi and Illinois Rivers (1989-2020).

We examined 30-year trends in the abundance and distribution of aquatic and floodplain vegetation, as well as human land uses in five study reaches of the Upper Mississippi River and one reach of the Illinois River using aerial photography collected in years 1989, 2000, 2010, and 2020. Permanently inundated area increased in all study reaches over the 30-year period. Increases ranged from 0.8% of study reach area in Pool 8 (73 ha) to as much as 6.5% of study reach area in Pool 13 (1,562 ha). Agricultural land use declined in the three study reaches where it was common (>35% of reach area). Agricultural declines ranged from 5.8% of reach area in Pool 26 (2,096 ha) to as much as 15.4% of reach area in the Open River reach (7,121 ha) and corresponded with a similar magnitude increase in permanently inundated area and semi-permanently inundated marsh classes. Total forest area declined in the four northern study reaches of the Upper Mississippi River. Forest loss estimates were on the order of 3.7% of study reach area in Pool 13 (905 ha), 2.1% of Pool 8 (364 ha), and 2.3% of Pool 4 (563 ha). Such losses represent 16.2%, 13.2%, and 10.9% of the total forest area in 1989 in Pools 13, 8 and 4, respectively. Permanently inundated area, wet meadow, shallow marsh vegetation, and mud were the main cover types that replaced forest cover in these reaches. In contrast to the decline in forest cover in the northern reaches, forest cover remained unchanged in the La Grange reach of the Illinois River and increased by 3.5% of study reach area (1,607 ha) in the southern Open River reach of the Mississippi River, mainly in former marsh vegetation and agricultural areas that were acquired by Federal and State agencies. The predominant changes observed across the study system (replacement of agriculture and forest area by permanently inundated area and semi-permanently inundated marsh classes) indicates that hydrological changes have been the main driver of change since 1989 throughout most of the Upper Mississippi and Illinois Rivers. Our study provides an example of changes in a regulated river system driven by regional-scale hydrological changes and local scale restoration actions, changes that could be compared against changes occurring in other large, regulated rivers across the globe.