River confinement and braiding loss in Canterbury region, Aotearoa New Zealand

Abstract

In relation to the wider concern that rivers in Aotearoa New Zealand have been narrowed by river control and land-use encroachment, and that iconic braided patterns are being lost, nine braided rivers from the Canterbury region were studied to compare river width, pattern type and braiding intensity between the mid-1900s and the present based on mapping from aerial images. Channel narrowing occurred along >90% (~490 km) of the length of the rivers studied, 375 km of which were historically braided. In total, the rivers narrowed by an average of 43% (48% for braided reaches). Coinciding with narrowing, braided reaches lost 1.3 channels, on average. Overall, 20% (over 100 km) length of the rivers recorded a change from braided to more simple pattern types including wandering (~18%) and single channel (~1%). The relationship of channel width and pattern change demonstrates the predictability of braiding change based on channel narrowing. As channels narrow, the likelihood of pattern simplification and change increases, although the amount of narrowing required to induce change depends on the initial width and pattern. Narrowing causes a reduction of braiding intensity even when the channel pattern remains braided. Based on these results, a predictor of channel pattern change and reduction of braiding intensity, based only on channel width and width change, can be applied to aid room-for-river management plans for conserving or restoring braided river morphologies. Channel width provides a general criterion for the prediction of braiding intensity and channel pattern threshold illustrating the scale-related effects on channel pattern.