Contextualizing Geomorphic
Heterogeneity
Geomorphic heterogeneity metrics pair well with descriptors of process space, utilization of process space, and geomorphic trajectory. Confinement, or process space (sensu Ciotti et al., 2021) describes the proportion of the valley bottom over which fluvial processes are active (e.g., the proportion of the valley bottom occupied by channels and floodplains). Process space utilization is the degree to which the river is actively reshaping the space available to it (e.g., the proportion of the channel and floodplain area occupied by channels either at a given time or cumulatively over a period of time). Rivers with greater process space can exhibit higher lateral connectivity, lower longitudinal connectivity, and higher spatial heterogeneity (Choné & Biron, 2016; Fotherby, 2009; Williams et al., 2020; Wohl et al., 2018; Wohl & Iskin, 2019). Because process space can regulate spatial heterogeneity, it can help to measure process space to aid in interpreting either spatial or temporal variability in heterogeneity metrics.
Although measurements of process space can identify constraints on the river corridor, measurements of process space utilization, or the degree to which the river is actively reshaping its fluvial process space, can identify deficiencies in the ingredients necessary to reshape the available space (e.g., flow, wood, sediment, etc.). Process space utilization can be measured by the proportion of the channel and floodplain area occupied by channels either at a given time or cumulatively over a period of time. Temporally, process space utilization can be measured as the turnover rate of the fluvial process space. Process space utilization can indicate the effects of different magnitudes of forcings (e.g., various flood magnitudes or durations), provide key context for observed temporal heterogeneity, or highlight hotspots of change that have caused changes in landscape scale spatial heterogeneity.
Finally, geomorphic trajectory (Fryirs et al., 2012; Mould & Fryirs, 2018; Surian et al., 2009), especially the trajectory of spatial heterogeneity metrics, can indicate whether the geomorphic processes that sustain heterogeneity are indeed active. If geomorphic heterogeneity is being sustained or increasing through time and the metrics used to infer geomorphic heterogeneity adequately reflect active geomorphic processes, then it stands to reason that geomorphic processes are active or even increasing in magnitude or rate.