Modeling formative floods in gravel-bedded rivers with bedfast ice
Gravel-bedded rivers are shaped during floods. Over time, certain floods do the most “geomorphic work” on river beds and banks by maximizing the product of sediment transport magnitude and frequency. The flood that does the most geomorphic work is known as the “formative flood” - it may not be the peak, but it reoccurs more often. However, it is unclear if this concept applies to freshet-dominated rivers in the Arctic. Gravel-bedded rivers in the Arctic continuous permafrost zone are occupied by river ice for 7-9 months each year. Freshet-dominated rivers in this region, like the Canning River, AK, receive a peak flood following snow melt while river ice can resist breakup for weeks, extending flood duration and magnitude. Still, the spring freshet occurs when hydraulic cross-sections are restricted by bedfast ice, limiting bed and bank exposure, but maximizing stage height. In contrast, summer floods generated by storm runoff occur when ice is absent, multiple times per year. Still, river bank and bed gravel is coarse and difficult to transport under low flows. We aim to investigate which flood maintains the Canning River’s hydraulic geometry, and more generally the hydraulic geometry of all ice-impacted, gravel-bedded rivers.
We explore if there is a formative flood for rivers that develop bedfast river ice with Basement V4. We focus on a 20 km long reach of the Canning River in Arctic Alaska, where we monitored break-up period from 2021 through 2024. We use USGS data for realistic peak discharges and ArcticDEM for surface topography, and field measurements of river ice thickness as primary model inputs. We assess geomorphic significance with metrics for potential sediment transport. To quantify the geomorphic significance, we compare bed and bank shear stresses produced by floods to thresholds for sediment entrainment and bank widening.
Initial results suggest that no realistic discharge can fill the bankfull channel or erode river banks when the channel is free of ice. Conversely, we find that these floods can sediment transport and bank erosion when bedfast ice persists. For the Canning River, the spring freshet is likely the formative flood. Our findings also show that the bedfast river ice can enhance bank erosion and might lead to wider rivers. The significance of river ice in this setting emphasizes that the stability of Arctic Rivers depends on river ice.