Measuring Temporal Dynamism

Temporal dynamism 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 mean turnover rate of in-channel landforms). Regardless of spatial scale, temporal heterogeneity is just as dependent as spatial heterogeneity 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 turnover time than one defined by low flow wetted channel, bars, early successional floodplain, and late successional floodplain.
There are two pieces of contextual information required to interpret measurements of 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. 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).