When the last glacier melted some 10,000 years ago, the Wisconsin River swelled to more than 100 times its present size and carried billions of tons of fine sand and gravel toward the Mississippi River. As the meltwater decreased, it deposited material in this valley to a depth of 300 feet, creating wide steps, or terraces, leading down to the present river bed. One of the upper terraces became a sand barren, named for its desert-like conditions, where evaporation exceeds rainfall and plants and animals have adapted to conserve water. Grasses grow many long roots, their leaves curl inward to reduce evaporation, and lizards and snakes stay underground during the day where temperatures may be up to 70 degrees cooler than on the sun-baked surface. This area is home to Wisconsin's only native cactus, prickly pear, whose green thorny pads are enlarged, flattened stems that both convert sunlight into food and store large quantities of water, while its four-inch yellow flowers bloom for only one day each in late June. On the lowest terrace, the Wisconsin's flood plain supports moisture-loving trees with shallow roots that get oxygen during floods but also make the trees likely to uproot during windstorms. A few miles north, bald eagles winter where a power plant's warm water keeps the river from icing over, and they can be seen perching in shore trees or swooping down to catch small fish.