High density topographic data
of large braided riverbeds (such as we have obtained with airborne laser scanning,
digital photogrammetry, and remotely sensed bathymetric mapping) reveal a
richness of morphological features that is easily missed by two-dimensional
imagery or ground inspection. Indeed, the classical air-photographic images
of braided rivers that highlight the binary pattern of wetted channels may
offer scant detail of the topography over which the water percolates.
We find in the Waimakariri study
reach that active braiding is largely confined to a belt that tends to meander
between broad, bank-attached bars that have the wavelength expected of single-row
alternate bars but have more subdued local relief.
The prominent channel-scale morphologic
features seen on these broad alternate bars are dendritic drainage networks,
which we infer are formed during relatively brief phases when flood flows
and freshes inundate the bars but introduce no or little new bedload.
Time-lapse video imagery shows
that during floods bedload transport along the braid belt is manifest by migrating,
lobate gravel sheets. The braiding pattern resumes when the receding flows
are draped over and re-work the topography of the stalled gravel sheets. Also,
the video records show that the classical braiding processes of braid-bank
scour and bar growth at difluences continue at normal flows and contribute
to a spatially and temporally intermittent flux of bedload.
