These findings can be explained by the geology of the study area, in particular the variation of the mineralogical composition of the bedrock from upstream to downstream.
The bedrock was characterized upstream by colluvium, calcareous crusts, encrusted pebbles, gypsum crusts, and Triassic rocks.
Regelation occurs because as the ice flows over a bedrock obstacle, the higher upstream pressure causes the ice to melt at the interface, because the melting temperature depends on pressure.
The ɛNd values of the river sediments are controlled by the bedrock values of the drainage upstream of the sample point, and the monazite content of the sediments because Nd is especially concentrated in this mineral (e.g., Garçon et al. 2014).
The lower ɛNd values in the downstream compared to the upstream are largely a function of this change in source bedrock and the fact that the river continues to gain sediment as it progresses downstream.
Channel storage is the main process controlling residence time in upstream conditions, where the riverbed is mainly composed of stones and bedrock.
In contrast to the alluvial reach upstream, Hells Canyon below the HCC is an exceptionally erosion-resistant, bedrock-dominated canyon landscape (Fig. 3).
We propose these reaches represent knickpoints, which developed as regional uplift raised tributary bedrock channels above the local fluvial baselevel during glacial intervals, and migrated upstream as the fluvial system was re-established during interglacial periods.
The proximity of a channel hard point such as bedrock or artificial grade control was also significant, indicating that channel enlargement increased moving upstream from grade control.
Wagga Wagga is upstream from the Riverina plain in the mid-catchment range of the Murrumbidgee River in an alluvial valley confined by low bedrock hills.
