It can seem counterintuitive to say that allowing water to flow freely will help reduce flooding. But removing dams, artificial channels and impermeable walls can help keep water from reaching homes and businesses.
In their natural states, rivers and streams meander through landscapes like wetlands and wet prairies. Some of their water filters into these areas, where it is held by plant roots and porous ground. During rainstorms, this floodplain habitat acts as a living sponge — soaking up water that can be slowly released back into streams.
Since colonial times, people have straightened stream paths, built walls and dams to control streams’ flow, and drained soggy land to use for agriculture or building. During heavy rainfall, water accumulates in these streams so quickly that it often spills over the tops of banks, dams or other control devices. Meanwhile, the surrounding land has been developed in ways that leave it too dense or hard to absorb such a rush of water. The result: flooding.
This is why, whenever Metro Parks and Nature acquires a new natural area, one of the first things staff scientists do is study its hydrology (water movement) and find ways to restore things to a more natural state.
“This is a common misunderstanding we encounter during restoration work,” said Metro Parks and Nature conservation program director Dan Moeller. “Often, we remove artificial waterways that may have been around for decades, which were draining water from the surrounding land. From the outside, it can look like we’re taking a beloved grassy meadow with a stream running through it and turning it into a muddy swamp. But wetlands are crucial habitat for wildlife, and they’re also an important piece of protecting the surrounding area from flooding.”
The area around Kellogg Creek in Milwaukie has experienced its share of flooding over the years, with major events in 1996 and 2015. During heavy rains, creek water collects behind the dam in what is locally called Kellogg Lake, even though it is not a natural lake but a manmade reservoir. Eventually, the water spills into surrounding neighborhoods.
“Right now, it’s a full bathtub,” North Clackamas Watershed Council executive director Neil Schulman said of the reservoir. “After the dam removal and restoration, it will be an empty bathtub. The creek will flow down to the Willamette, and the restored wetlands will catch floodwaters before they make it into someone’s basement.”