Temporal Geomorphic
Heterogeneity
Geomorphic heterogeneity includes both spatial and temporal components. While spatial heterogeneity describes variability in geomorphic units from one place to another, temporal heterogeneity describes variability in geomorphic units through time in a single place. Temporal heterogeneity is simply the rate at which geomorphic units change, or turnover, from one unit to another.
Some degree of disturbance and turnover of geomorphic units is usually a prerequisite for sustained spatial heterogeneity, as disturbances rearrange geomorphic units (Rice et al., 2012; Townsend, 1989) and the riverine habitat mosaic (Arscott et al., 2002; Stanford et al., 2005; Ward et al., 2002; Willig & Presley, 2018). Rivers that lack either driving forces (e.g., geomorphically effective flows) or forms (e.g., wood that increases roughness) that generate disturbance and turnover of geomorphic units tend towards a more homogenous state (flow regulation is a good example; Gendaszek et al., 2012; Graf, 2006). However, constant, high-magnitude disturbance is not necessary to sustain a heterogeneous character — different river systems (e.g., monsoon-dominated versus snowmelt-dominated) and different portions within a river system (e.g., channel versus floodplain) will require different turnover rates, or different disturbance regimes, to sustain different aspects of spatial heterogeneity.
Although one-time alterations, perhaps due to river restoration (e.g., Stoffers et al., 2020) or unusually high-magnitude flows (e.g., Gendaszek et al., 2012), can generate high spatial heterogeneity, estimating turnover rate can help determine whether those short-term gains are likely to be sustained, or whether an alternative state has been reached (Livers et al., 2018; Phillips & Van Dyke, 2016). For example, comparing post-restoration to pre-restoration turnover rate could indicate restoration effects on the overall erodibility of the valley bottom. Turnover rate might indicate whether that restoration simply made the landscape more heterogeneous or has reactivated the processes needed to sustain that heterogeneity.
Measuring Temporal
Dynamism
Temporal heterogeneity can be expressed as a turnover rate (change per unit time) or its reciprocal, turnover time (time required for the entire landscape or portion of the landscape to change), both typically derived from many observations of change. Temporal heterogeneity can be calculated at the level of individual landforms, analogous to class level spatial heterogeneity metrics (e.g., floodplain turnover; Beechie et al., 2006; O’Connor et al., 2003) or for entire areas, analogous to landscape level spatial heterogeneity metrics (e.g., the turnover rate of all in-channel landforms). It is important to note that different portions of the river corridor may be expected to change at different rates, so measuring temporal heterogeneity over the entire river corridor may be misleading, whereas measuring it separately, for instance, for active channels versus floodplain surfaces versus terraces, may better represent the real propensity of the landscape to change.
Regardless of spatial scale, the interpretation of temporal heterogeneity metrics depend strongly on the definition of geomorphic units. In a meandering river, for instance, a geomorphic unit schema defined only by channel and floodplain units will have a longer average turnover time than one defined by low flow wetted channel, bars, early successional floodplain, and late successional floodplain, as the more detailed geomorphic unit schema will be more sensitive to frequent changes, such as vegetation succession.
Two pieces of contextual information are required to unbiasedly assess temporal heterogeneity: observation frequency and disturbance frequency. Observation frequency dictates the maximum detectable turnover rate, as geomorphic units cannot be observed to change more times than there are observations. Observations should be timed appropriately to the frequency with which geomorphic units are expected to change. For example, observations every 5 years will only provide a minimum estimate — likely a dramatic underestimate — of the turnover rate of fast-changing geomorphic units, like grain size patches in a gravel-bed river. Disturbance frequency, or the frequency of events that can change geomorphic units, sets the expectation for maximum potential temporal heterogeneity. A system with very low in-channel geomorphic unit turnover rate might be behaving just as expected if there have been no geomorphically effective flows in the period of measurement, but that same turnover rate over a period of multiple major floods would likely indicate a channel boundary that is very resistant to change, assuming observations were timed appropriately. Dating of floodplain strata or the use of historical imagery can be effective ways of extending the period over which temporal heterogeneity is measured, which can be key to measuring turnover rates for slow-changing geomorphic units. It stands to reason that normalizing turnover rate by dividing it by disturbance frequency can be a useful way of comparing across sites with similar geomorphic processes but differing rates of those processes (e.g., different flow regimes).