Sedimentary characteristics and morphologic change of till-bedded semi-alluvial streams: Medway Creek, Southern Ontario, Canada

Abstract

We describe the first detailed reach-scale study of an incisional till-bed river. Our analysis focusses on boundary till characteristics, bare till patch features, annual erosion rates, bedform dimensions and spacing, and grain size distributions of bedforms. Results show that till exposures constitute a relatively small portion of the bed and that till erosion rates are relatively high compared to bedrock rivers, although highly variable between till patches and within patches. The bedforms are not well organized in terms of spacing and show high morphologic variability. The sediment forming the bed is poorly sorted, and grain sizes of the bedforms show high variability ranging from fines to large boulders, although gravel contribution from the till to the alluvium is relatively small. We found evidence of some in situ and transport rounding of till clasts. As expected, riffles and steps are coarser while glides and pools are finer-grained. Sedimentary stability metrics show that riffles are unstable, while pools and glides are more stable. These results indicate that the bedform morphology and sedimentology till-bedded rivers differ substantially from their alluvial and bedrock counterparts in a variety of ways. Consequently, we recommend that semi-alluvial rivers be differentiated from their alluvial and bedrock counterparts in future channel classifications. Such a practice will be useful for the river research and practitioners' community to gain the appropriate research tools needed for assessment, management, and restoration practices for these rivers.