Green River Lowland: Glacial Action Shaped Unique Sandy and Wetland Habitats

Walking the Green River Lowland reveals what we believe the landscape looked like thousands of years ago, and how forces of nature formed a diverse mosaic of habitats, including sand dunes and some of the flattest parts of Illinois. Yes, there are natural sand dunes in Illinois.

All those natural occurrences, spurred on by glacial retreat some 19,000 years ago, created the Green River Lowland, which covers roughly 832 square miles in parts of Bureau, Henry, Lee and Whiteside counties in Illinois. The Green River Lowland is a section within the Grand Prairie Natural Division of Illinois, which includes more than 20,000 square miles.

“What we see today in the Green River Lowland Section is a result of a continental ice sheet that forced the Mississippi River westward to its present course and the filling of the deepest bedrock valley in Illinois with glacial debris,” said John C. Nelson, a recently retired Natural Areas Protection Specialist for the Illinois Nature Preserves Commission.

It was on top of this buried valley that the poorly drained Green River Lowland formed under the influences of water (liquid or solid) and wind. The retreating glacial ice left vast shallow lakes and an uneven and poorly drained landscape in the area. Winds blew across vast shallow lakes causing sediments to be evenly distributed on lake bottoms. During winter when the lakes were frozen, winds carried sands across the ice. Over time, portions of the lowlands accumulated more sand than others and, in some places, sand dunes formed that tower over the surrounding lowlands. These dunes and sheet deposits are a defining characteristic of the Green River Lowland.

After the last glacial era some 12,000 years ago, Illinois’ climate became warm and dry, which initially favored the establishment of grasslands across most of Illinois. As the climate again changed and became cooler and more temperate, trees were favored, although even with favorable conditions trees could not easily take over the vast expanses of grasslands. Factor in an Aboriginal fire regime and tree establishment could only happen where fires were less frequent or intense. In the Green River Lowlands, this resulted in a mosaic of various grasslands, interspersed with timbered groves and oak savannas.

“Fire was a major factor that limited the establishment of trees, particularly on the sandy well-drained uplands,” Nelson said.

“The habitats within the Green River Lowland range from the wettest of the wet to the driest of the dry and everything in between,” he said. These varied habitats provide home for a wide variety of plants and animals, some of them confined to just a few places in Illinois.

For example, “an environment with sand surrounded by wetlands is ideal for many reptiles,” Nelson said. “They can hibernate and lay eggs in the sandy upland areas and find food in the wetlands. We still find high concentrations of reptiles in the Green River Lowland, including the state-endangered Blanding’s turtle, and state-threatened ornate box turtle and western hognose snake.”

Private and public ownership and management of lands within this region today help support habitats for rare reptiles, amphibians, birds, butterflies and other animals, as well as rare plants, including poppy mallow and sedges.

Other important natural features were the ‘Great Swamps’ that formed in the most poorly drained areas.

“The wildlife diversity of the swamps must have been incredible,” Nelson said. Two of the largest swamps, named Inlet and Winnebago, are nearly entirely drained today. Historically they attracted all sorts of plants and animals. In 1875, American Naturalist George Henry Perkins (1844-1933) described in The American Naturalist the diversity to be found in the Illinois lowlands. According to his account, after spring, when the high waters subsided, ponds were bordered by mud and sand over which hosts of large, colorful snails crawled. He wrote of abundant food that attracted ducks, plovers and herons along the pond margins where sedges and grasses grew. Describing such rare prairie plants as the Queen-of-the-prairie, endangered in Illinois today, Perkins noted “From early summer to late autumn, many a rare and beautiful flower is seen.”

The sandy regime and copious wetlands kept farmers away from this part of the state for a long time. In fact, these were the last places on the landscape in that region of Illinois to be drained for farming, Nelson said. But over time, the draining of wetlands, the introduction of irrigation and the suppression of fires took its toll on the native landscape.

Today, much smaller versions of the pre-settlement landscape still exist—and that’s thanks to private and public landowners, including the state and nonprofit organizations, Nelson explained.

Private landowner Greg Wahl, for example, owns roughly 386 acres called Hahnaman Sands Prairie and has purchased two nearby farms to expand and restore it to its pre-settlement glory.

“This is shortgrass, sandy prairie with wetlands in between, providing for a large number of rare species,” Wahl said. “I do this to protect biodiversity. This land is my idealistic purpose for living the rest of my life, to make a positive difference on Earth.” Hahnaman Sand Prairie was listed as an Illinois State Nature Preserve in 2014.

Nearby, Green River State Wildlife Area, owned by the Illinois Department of Natural Resources, is “a living fossil of what was the much larger Winnebago Swamp,” according to Nelson. There, visitors will see various types of wetlands, a gently rolling terrain with sand deposits and birds favoring this habitat, including lark sparrow and blue grosbeak. White-tailed deer, wild turkey and mourning dove hunting is allowed at Green River.

The state has been clearing trees and brush at the wildlife area to maintain the dry, sparsely vegetated areas. In addition, pine trees planted long ago for timber and soil erosion are being removed so they don’t shade prairie grasses.

“Illinois Audubon Society is the leader right now in helping to acquire and protect land in the Green River Lowland,” Nelson said, mentioning Amboy Marsh Nature Preserve and the Gremel Nature Preserve, both near Amboy. The organization is adding more land and restoring it, he said.

A self-guided hike at Amboy Marsh Nature Preserve will take visitors up to sand dunes, past sand prairies and oak sand savanna, and down into wetlands and sedge meadows. In the spring, snakes like to warm themselves in sandy, sparsely vegetated areas. If it gets really hot, they’ll look for shade. Nelson has found green snakes, tiger salamanders and eastern newts at Amboy Marsh and botanist Linda Curtis has catalogued a host of rare sedges there.

“Landowners, such as the Illinois Audubon Society, are doing all this wonderful work and they know what they’re doing,” Nelson said. “They’re the future of conservation in Illinois. Government can’t do this alone.”

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Sheryl DeVore writes environment and nature pieces for regional and national publications and has had several books published, including “Birds of Illinois” co-authored with her husband, Steven D. Bailey.