Channel Response and Recovery to Extreme Flood Events in Alluvial Rivers.

Extreme floods play an important role in modifying alluvial river channels. However, the impact of single floods on the channel depends on many factors, including peak flood discharge and duration, presence and type of riparian vegetation, and the volume and calibre of sediment supplied to the channel. Yet, it is not well understood how flood responses are modulated by sediment and vegetation.

In this study we exploit data from the Nicola River, a major tributary of the Thompson River in southcentral British Columbia, Canada, draining about 7,600 km2. The study area experienced intense rainfall in November 2021, causing slope destabilization and unprecedented flooding. Substantial channel adjustments were observed due to bank erosion and the reworking of in-stream sediment deposits. The 2021 flood on the Nicola River provides an opportunity to compare the recent flood with past events.

We collected high resolution historic aerial photographs for the Nicola main stem dating back to the 1960s. Stereo pairs are utilized to calculate digital elevation models, which are used to build a detailed time series of morphological adjustments over the last 60 years. Additionally, we use LiDAR derived digital elevation models from 2018 and post flood from the years of 2021 and 2022. Using a list of indicators, we quantify channel adjustment and recovery, including channel width and changes in volume of in-stream sediment storage.

Our analysis will provide insights into morphological adjustments to a range of different magnitude flood events within the last 60 years.