From parking lot to floodplain — 14 acres returned to the Cedar River

The Cedar River Conservancy bought a failing business park for the land under it. The buildings were incidental; the point was that the whole parcel sat inside a mapped floodway that the river had been denied access to since 1968.

Removing fourteen acres of asphalt is straightforward. Deciding what elevation to leave behind is not.

Challenge

Two generations of fill had raised the site around six feet above its original floodplain grade. The river ran fast and straight past it in a channel that had incised nearly a metre since the levee went in, taking the water table down with it and stranding the remnant cottonwoods on the north edge.

Simply lowering the ground would have created a stagnant pond rather than a functioning floodplain. We needed the site to flood, drain and dry on the river's own schedule.

Solution

We surveyed the pre-1968 terraces from historical aerials and rebuilt them, keying three benches into the bank at elevations tied to the two-year, ten-year and hundred-year flows. The lowest bench wets several times each winter; the highest has been under water once since completion.

Six hundred tonnes of crushed concrete from the demolished slabs became the base of the new access road, which kept the material on site and out of the haul budget. Willow and red-osier dogwood went in as live stakes rather than containers — cheaper, faster to establish, and they root into the bank rather than sitting on it.

Two winters on, the gravel bar the river built for itself in the first season is now the most biologically active part of the site.