Gravel clast size variations in the Tasman River, Mt Cook, New Zealand

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Browne, G.H.; Barrell, D.J.A. 2002 Gravel clast size variations in the Tasman River, Mt Cook, New Zealand . Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences science report 2002/12 31 p.

Abstract: Gravel sizes were recorded at seven transect sites within a 21 km long reach of the Tasman River floodplain during March 2002 to determine downstream changes in clast size. The Tasman River, near Mt Cook in the central South Island, is 24 km long, extending from Tasman Lake at the terminus of the Tasman Glacier, downstream to Lake Pukaki. Clast size was determined by measuring the 3 axial dimensions (L-longest, I-intermediate, and S-shortest axis) of at least 100 clasts at each of the seven stations. Stations were spaced between 1 and 5.3 km apart. Site selection was influenced by ease of access to the floodplain surfaces. Transects were undertaken as close as possible to the middle of the Tasman riverbed to avoid localised inputs from tributary catchments. The data were also used to determine downstream changes in clast sphericity and form. The results indicate a generally systematic decrease in clast size down the Tasman River, from the terminus of the Tasman Glacier to Lake Pukaki. Intermediate clast data, as a proxy for mean clast size, indicate that clasts entering Lake Pukaki are approximately 25 mm in diameter, compared to 105 mm in the initial reaches of the river. One of the observation stations displayed a finer-than expected clast size, and we interpret this to reflect an over-representation of fan material from nearby tributaries rather than the mainstream Tasman River sediments. Sorting of clasts also improves systematically downstream for all axial dimensions (L, I, and S). Clast form is bladed throughout the length of the river studied, but there is an overall decrease in the percentage abundance of bladed clasts downstream, with a corresponding increase in cubic platy, cubic bladed and cubic elongate forms. We suggest this relates to mechanical breakage of the clasts downstream, into more compact forms. (auth)