Research Brief: Sediment Transportation from a Great Lakes Estuary

Freshwater estuaries are important habitats in lake ecosystems, harboring biodiversity and moving sediment between waterbodies.¹ Within the Great Lakes, many of the largest estuaries have become major population centers.

These are heavily urbanized with sediment buildup and dredging, limiting their natural cycles and impacts on lakes.¹ However, less is known about unimpacted estuaries. Especially how flooding and sediment movement impact the estuaries, their surrounding wetlands, and the lakes they drain into.

A 2025 study in the Journal of Great Lakes Research examined natural flooding and sedimentation in a Lake Superior estuary. It sought to understand how water levels affect estuarine floods and sediment movement, and how the results shape surrounding ecosystems.¹

Methods

The study was conducted in the Bad River estuary along Lake Superior’s Wisconsin shore. The Bad River and the Kakagon River feed the estuary.

Estuary water level and streamflow data were collected from USGS gauges. Historic water level was pulled from datasets from 2014 onward, whereas streamflow data was compiled from 1948 to 2023. Lake Superior’s historic water level was taken from NOAA datasets, also from 1948 to 2023.

The estuary was mapped over the years with lidar.¹ This data was compiled for 2015 and 2019, to compare topography before and after major floods in 2016 and 2018. Bathymetry data were collected in the field multiple times at each river since 2015 using side-scan sonar.

Satellite imagery was used to confirm the spatial extent and timing of inundation and sediment flow from the Bad River into the estuary, as well as snow melt events. These images were used to validate statistical analysis on flooding in the estuary.

Water level and stream flow data were used to estimate how often the two rivers flooded between 2012 and 2023, and how often those floods led to river connection, an important aspect of sediment transport and physical estuary change.

Water level gauge data and statistical analysis were used to estimate broad physical estuary changes due to inundation from 2016 to 2023, along with a finer-scale study using data from April to May of 2023.¹ The study also estimated inundation changes specifically following the two floods.

GIS was used to map changes from sediment erosion and deposition from 2015 to 2019. Finally, the study estimated changes in the maximum depth of the Bad River channel between 2015 and 2016, and from 2016 to 2021.

Results

The large estuary is geomorphologically varied, with diverse vegetation and a mouth that opens directly to the lake and into a bay with some small barrier islands.¹ The upstream watershed is relatively undeveloped.¹

Lake Superior water levels have varied steadily with the seasons, but high pulses are reflected in elevated river levels, leading to short-term inundations in the Bad River estuary of up to 45%.¹

However, floods like those in 2016 completely inundated the estuary.¹ Notably, the Bad River was sediment-laden earlier than the Kakagon River, which received sediment only after flood waters connected the two.¹ Sediment stayed in the estuary longer near where the Kakagon empties into Superior.¹

These satellite images generally validated the statistical analysis used to estimate flood extent.¹ Moreover, the satellite showed that longer ice extent increased inundation time and sediment build-up in the estuary, even from a small flood.¹

These images, combined with data from the water level gauges, showed that when Lake Superior reached levels above 184-185 m, sediment-laden floodwaters from the Bad River began spreading across the rest of the estuary.¹

“Lake level fluctuations and related coastal flooding in the Bad River estuary. (a) Graph showing lake level and percent of estuary inundated; inundation (light blue for lake levels) of (b) 183.8 m NAVD88; (c) 184.1 m NAVD88; and (d) 184.2 m NAVD88 extend of wetted land. Approximate estuary boundary is indicated on the maps by the black line. Base map imagery credits: ESRI and Maxar.” (Credit: Fitzpatrick et al., 2025)

The geomorphic mapping characterized the estuary relatively well, although not at a fine scale.¹ Both river channels remained relatively unchanged in depth during the study timeframe. However, bathymetry estimated small net sediment deposition in both rivers.¹

Nonetheless, the analysis showed that sediment depth ranged from below 22 cm to 80 cm during different inundation events.¹ The deepest sediment remained in the river backwaters or was deposited into Lake Superior, rather than spreading evenly throughout the estuary.¹

The study estimated net positive sediment deposition in the estuary throughout the study period.

Overall, the study showed that lake levels, river floods, and ice all led to sediment movement within the estuary. With climate predictions suggesting more floods, the estuary and Lake Superior mouths may become more sediment-laden.¹

The study suggests that this could change the lake’s nearshore geomorphology, including water quality, vegetation type, and fish habitat.

Sources

1. Faith A. Fitzpatrick, Angus Vaughan, Eric D. Dantoin, Shelby P. Sterner, Paul C. Reneau, Collin J. Roland. Effects of river floods and sedimentation on a naturally dynamic Great Lakes estuary. Journal of Great Lakes Research, Volume 51, Issue 1, 2025. https://doi.org/10.1016/j.jglr.2024.102458.