The western reaches of Minneapolis including Theodore Wirth Park, wherein lies the Eloise Butler Wildflower Garden, and including the western chain of lakes, are situated on or quite near the ancient streambed of the Interglacial Mississippi River. Most of the Twin Cities is covered by glacial drift and glacial moraine material which constitutes the hilly landscape. With the hugh volume of melt water flowing southward from the retreating glaciers of the last great ice age (the Pleistocene era), various river channels were carved in the land. Four major glacial advances covered the Twin Cities - what are known as the Nebraskan, the Kansan, the Illinoian and finally the Wisconsin ice sheets. In between the advances of the different ice sheets, one channel of the ancient Mississippi River was created in the area of the Garden and the Minneapolis Chain of Lakes. It was only after the retreat of the Wisconsin ice sheet and the subsequent drainage of Glacial Lake Agassiz and Glacial Lake Duluth that the present Mississippi channel north of Fort Snelling became the river we know today.
The small rolling hills of the Garden are typical of a glacial drift area. The maximum elevation point is 921 feet at the far hill in the Upland Garden. This elevation is about the same height as other high points elsewhere in western Minneapolis and Golden Valley. The ancient riverbed is most noticeable just south of the Garden and just east in the area known as Bryn Mawr Meadows in the area where the former railroad yard existed. The railroads found the old streambed most convenient for tracking.
The wetland area of the Woodland Garden was spring fed when Eloise Butler and her fellow teachers selected the area and just to the north, outside the current Garden perimeter, was the Great Medicine Spring. These springs have all been adversely affected by changes in the recharge of ground water over the years due to metro development. The springs in the Garden were the first to be affected. Following the droughts of the 1930s, they never fully recovered. In 1939, Garden Curator Martha Crone mentions in her annual report that a new spring was taped in the wetland area near the open pool in order to keep the supply of water uniform. In 1947 she requested that a city supplied water system be brought into the Garden, not just for the Woodland area but also for the newly added Upland Garden. It was installed that same year in the Upland Garden but it was years later before it was piped to the Woodland Garden. Last to be affected was the Great Medicine Spring. Following the construction of Interstate 394 just south of the Garden, the spring declined much further and today, even after an attempt by the Friends to resurrect it, the spring has been abandoned.
More detail on the Springs in the area at this link.