Why are many former glacial spillways wetlands today?

Many former glacial spillways—valleys carved by meltwater during the Pleistocene Epoch—are wetlands today because of the specific geological and hydrological conditions left behind by retreating glaciers.

As glaciers melted, they carved wide, flat valleys that were often filled with thick sequences of sand, gravel, and other glacial outwash deposits. These valleys are often occupied by modern rivers that appear "underfit," meaning they are significantly smaller than the massive valleys they flow through. Because these valleys are broad and flat, they frequently contain floodplains where water can easily spread and accumulate.12

Furthermore, the uneven scouring of the landscape and the melting of buried ice blocks created depressions, such as kettle lakes, which trap surface water. In many areas, the combination of these low-relief, poorly drained surfaces, high water tables, and the presence of fine-grained sediments creates the ideal environment for the development of wetlands, including marshes, swamps, and peatlands, which persist in these postglacial landscapes.13