Monitoring river systems with repeat aerial photography is a powerful tool although the temporal resolution of surveys is rarely performed at anything better than annual time-scales. In recent years, a variety of low-cost aerial platforms for acquiring aerial photography have emerged. While these economical options may facilitate more frequent repeat surveys, the accuracy of imagery needs further consideration.The accuracy of imagery obtained from a Lighter-Than-Air Blimp is investigated through two simple experiments. The first looks at the geospatial error of aerial photographs derived using five geometric transformation models, and the sensitivity of the photo registration quality to various ground control point (GCP) configurations and densities. At high GCP densities, higher order polynomial transformation models provide the highest quality registrations. However, at more modest GCP densities (i.e. 19-28 GCPs HaÀ1 ), simple aerotriangulation and 2nd order polynomial transformation models perform modestly, resulting in registration errors at standards equal to or better than obtained with conventional aerial photography (e.g. 0.5-1 m). The quality of image registration is highly dependent on the configuration of GCPs. In a second experiment, the practical utility of producing a mosaic of blimp acquired imagery is explored over a kilometre long braided reach. Even at relatively low GCPs densities (e.g. 9 GCPs Ha À1 ), a mosaiced aerial of the entire reach can be produced of adequate quality to support bar-scale mapping of patchscale features. The survey required less than a single day of field work and laboratory processing, and presents a cost-effective alternative to traditionally commissioned flights.

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Systematic grain size measurements were recorded for all geomorphic units in 41 cross sections spaced 400-800 m apart along the lower course of the Ngaruroro River. Although the surveyed bed slope has a uniform grade through this reach, valley width imposed by terrace margins increased markedly, with a notable increase 6 km down-reach. Mean grain size and D 95 diminution coefficients of 0.052 and 0.073 were determined for the 19 km study reach. Marked differences in downstream fining trends were evident for the zones upstream and downstream of 6 km, with significant fining upstream of 6 km and more subdued fining downstream of 6 km (D 95 diminution coefficients of 0.109 and 0.017 respectively). The increase in active channel area associated with the increase in valley width is considered to mark a process control shift from competence-limited to capacity-limited conditions. Selective entrainment rather than abrasion is inferred to be the primary mechanism for downstream fining. Lateral confinement also affects patterns of geomorphic units. The competence-limited environment (upstream 6 km) comprises stabilised bars, stabilised gravel sheets and gravel sheets while the capacity-limited environment (downstream of 6 km) is characterised by significant lateral variability of gravel sheets, high flow chutes, ridges, ramps, platforms, stabilised bars and stabilised gravel sheets. Findings from this study highlight the importance of lateral confinement as a control on river processes and forms.