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).