Floodplain Focused: Insight into Achieving Great River Restoration, Without the River

Floodplain Focused: Insight into Achieving Great River Restoration, Without the River.  

Sydney Salzwedel1*, Michael Lighthiser2  

1Biohabitats, Denver, CO, USA; ssalzwedel@biohabitats.com  

2Biohabitats, Denver, CO, USA; mlighthiser@biohabitats.com   

When using a process-based approach to river restoration, the goal is to address the root causes of degradation. While we often think of river restoration focusing on the river itself, sometimes the key to restoring a system’s processes instead lies in the floodplain, an essential component of the riverscape. With this project, our goal was to reestablish connectivity, diversity, and variability, while not disturbing the well-functioning aquatic ecosystems of the river itself.  

This project is located on the South St. Vrain River, just outside the town of Lyons, CO. It was one of the front range waterways drastically impacted by the 2013 floods, where nearly 18 inches of rain fell, and floodwaters surged to levels comparable to a 0.1% annual exceedance probability. The project is on the Hall Ranch property, where, during the flood, an undersized bridge on the South St. Vrain caused water to back up and drop huge amounts of sediment across the floodplain. The river cut a new path through the deposited sediment, however, was never able to reconnect to the floodplain given the elevation differences.  

While the South St. Vrain riverbed provides great aquatic habitat, the hydrologic disconnect from its floodplain created a distinct lack in riparian habitat. The goals of our project also included restoring a more appropriate sediment regime – one where flows could frequently leave the river’s banks, slow down, and deposit excess sediment instead of washing it all downstream. This process also relies on floodplain connectivity.   

The need for connectivity was addressed with multiple approaches: excavation of remaining sediment from flood deposits, grading of side channels to activate at certain flow levels, and the installation of a variety of wood structures to provide both hydraulic and habitat benefits. This was all planned around existing high-functioning habitats, including the river.  

When designing for restoration, look valley-wide for the root cause of process degradation. A successful and sustainable river restoration project might be achievable with a focus on floodplain.