Heterogeneity Aspect |
Heterogeneity Aspect |
Typical Application Level |
Meaning |
Spatial Heterogeneity
|
Diversity (richness and/or evenness)
|
Landscape
|
Richness describes how many unique geomorphic unit types exist in an
area. Evenness describes the relative abundance of each unit type, with low
evenness indicating that some units take up most of the landscape, and
high evenness indicating that most units take up the same proportion of
the landscape.
|
|
Spatial Configuration |
Class or Landscape |
Describes the geometry
and arrangement of individual or groups of geomorphic unit types,
including, but not limited to, their subdivision, edge density, and
aggregation. |
Temporal Heterogeneity |
Temporal Heterogeneity |
Class or Landscape |
Describes the rate at which geomorphic units change, or turnover
rate. |
In conclusion, we suggest eight considerations when evaluating geomorphic heterogeneity in river corridors:
There is no single degree of geomorphic heterogeneity that defines a well-functioning river corridor. Geomorphic heterogeneity expectations (or, for restoration, goals) will differ depending on active geomorphic processes, process space, disturbance regime, the metrics and geomorphic unit schema being applied, and what people value about a river corridor.
Carefully design geomorphic unit schemas to address specific objectives. To evaluate a specific process, consider what components of the river corridor that process tends to alter, and how to define those components in a way that is relevant for that process.
Select heterogeneity metrics that will describe the characteristics of geomorphic unit assemblages that relate to specific objectives. Different metrics are sensitive to different characteristic, such as spatial configuration, evenness, or turnover. Choose metrics that conceptually represent characteristics of interest, then check that they match qualitative observations across a range of possible conditions.
Interpret spatial heterogeneity metrics in the context of potential heterogeneity, fluvial process space, and the processes that drive geomorphic change. Measurements of spatial heterogeneity should usually be contextualized by expectations about the maximum level of heterogeneity a river corridor might be expected to achieve, based on available fluvial process space, flows of water, wood, and sediment, and ecological function.
Use multiple metrics to achieve more holistic descriptions of heterogeneity. For instance, evenness and subdivision metrics can together describe overall valley bottom heterogeneity better than either metric alone.
Set expectations for heterogeneity that are specific to the system in question. Applying the same geomorphic unit schema across rivers with very different characteristics and active processes may yield misleading comparisons unless the schemas and metrics applied are comparable. It may be more appropriate to compare heterogeneity metrics based on unique geomorphic unit schemas specific to each river system, but that reflect analogous processes.
Frame expectations of heterogeneity based on scale. Landform spacing is scale-dependent (e.g., pool-riffle spacing depends on channel width; Gregory et al., 1994), meaning that so too are heterogeneity metrics that describe them (e.g., compare the narrow versus wide channels in Figure 1). Similarly, measurements of turnover rate depend on the observation interval relative to disturbance frequency. Varying spatial or temporal scales will produce different and potentially incomparable heterogeneity metrics.
Provide context for spatial heterogeneity using temporal heterogeneity. Although spatial heterogeneity alone can be useful, it may produce misleading conclusions without the context that comes from evaluating turnover rate. This is especially important when the sustainability of a heterogeneous state is in question.
Geomorphic heterogeneity, or the spatial and temporal variability in geomorphic units, is a useful tool that allows investigators to infer geomorphic processes and quantify characteristics hypothesized to regulate those processes. By applying the concepts discussed here, we hope that investigators can continue to develop novel and effective applications of geomorphic heterogeneity to improve our ability to describe river forms and processes.