Channel evolution controlled by valley configuration during 70 years in a severely erosive catchment: Mangaoporo River, New Zealand

摘要

We examined the channel-evolution process in a 19.5 km course in the Mangaoporo River (catchment area: 72.6 km(2)) in the East Coast region of North Island, New Zealand over 70 years using aerial photographs and cross-sectional survey records. The catchment experienced expansive deforestation during the late 19th and early 20th centuries, which accelerated the growth of the gully complexes. The manner of the evolution was controlled largely by the underlying lithology that determined the valley configuration. In the upstream confined section, where hillslope and channel processes were closely coupled, continuous sediment supply from gully complexes gradually converted the channel course from a narrow single-thread to wide braided reaches, or from sediment transfer to storage zones. This change was accompanied by a decrease in unit stream power (USP). In 1939, the USP was above 200 W/m(2) in most of the upstream section, while 40% of the USP values were near or below 50 W/m(2) in 2012/2013. Because a narrow reach situated at the end of the storage zone limited the amount of sediment travelling further, the downstream reaches were gradually disconnected from the upstream section, and the channel forms changed more moderately than in the upstream section. This reduction in sediment travelling with distance transformed a once-braided reach at the end of the course to development of a supply-limited condition with time. The overall channel evolution was accelerated by major rainfall events, including prolonged rainfall in 1956 and Cyclone Bola in 1988, which activated gully complexes and contributed to subsequent channel widening, particularly in the upstream section. Although reforestation starting in the 1980s contributed to deactivation of gully complexes after 1988, the difficulty in controlling their total remission together with the lower USP values of the river in 2012/2013 compared to those before the previous major wet events indicate that the river has been evolving in an irreversible direction, triggered by land use changes of more than a century ago.