Updated suspended sediment yield results for the Waikato region

Waikato Regional Council (WRC) has monitored riverine suspended sediment at 26 sites, and the monitoring period ranged between 6 and 33 years. This study reports on average annual suspended sediment yield, and time trends where possible, up to 2022 and updates previous reporting published by Hoyle (2014).

Average annual suspended sediment yield calculated using the suspended sediment concentration-discharge rating and the full flow record for each site ranged between 0.7 and 239.5 Kt/yr. The highest yield was recorded on the Waikato River at Rangiriri. Accounting for catchment area, average annual specific sediment yields ranged between 7.9 and 176 t/km2/yr. The highest suspended sediment yield was recorded at the site in the Matahuru catchment @ Myers, a tributary of Lake Waikare, which also had the highest mean suspended sediment concentration of 101 g/m3. The suspended sediment concentration-discharge rating and the annual flow record was used to calculate annual suspended sediment yields for each site. Annual yields showed more than a tenfold variation for many sites. This large variation was brought about by inter-annual variability in weather patterns and events, especially the occurrence of significant storm events in certain years.

Time trend analysis was calculated using residuals from the relationship between sediment concentration and the sediment concentration-discharge rating and plotted against time. Only 11 sites had sufficient data to undertake time trend analysis, and of those sites, 1 displayed no trend, 6 an increasing trend, and 4 a decreasing trend.