ELEMENTAL FLUXES AND SEDIMENT YIELDS FROM NEW ZEALAND RIVERS

Recent work of Milliman and Syvitski (1992) showed the importance of small mountainous rivers, previously neglected in calculating global geochemical budgets, as sources of particulate matter fluxes to the world’s oceans. Three watersheds on the western slope of the Southern Alps, draining into the Tasman Sea, were instrumented for automatic collection of water samples during hydrologic events in 1998-99. These watersheds are hosted in highly metamorphosed schists in a compressional area near the Indo-Pacific plate boundary, a rapidly uplifting region. Uplift rate is estimated to be 12 mm yr^-1 and the denudation rate 11 mm yr^-1. The annual average rainfall is 6500 to 10,070 mm yr^-1. Precipitation is highly influenced by orographic effects. Analysis of particulate and dissolved major, minor and trace element concentrations by XRF, ICP and ion chromatography from our samples allowed for calculation of chemical and sediment yields from the Hokitika, the Cropp and the Haast Rivers. The Cropp watershed, the smallest of the three studied, had an area upstream of the gauge of 12.2 km² and a maximum measured sediment load of 77,000 mg L^-1. The Cropp’s annual suspended sediment yield was 32,120 tons km^-2. Comparison of these results with published results from other mountainous rivers show far greater suspended sediment loads from these New Zealand streams. Suspended sediment yield of the New Zealand rivers was four to five orders of magnitude greater than that of the Nile, for example. Compared to world major river particulate concentrations (Martin and Meybeck, 1979), particulate matter from the Cropp had enrichments of as much as threefold for soluble elements like Na, K and Mg, but were similar to world averages for refractory elements like Si and Ti.