The Waikato River was flowing a muddy brown today after slips and scouring in tributary streams.
Environment Waikato flew over the area this morning to find the source of the unusually heavy mudflow and found it was coming from the Karapiro Stream immediately upstream of Cambridge, which drains the Te Miro and Whitehall hills. Further upstream the Waikato River was normal.
The Mangaonua Stream – which also drains from the Te Miro hills – is also muddy.
Resource Information Group Manger Tony Petch said there appeared to have been extremely heavy thunderstorms yesterday afternoon which dumped a large amount of rain over a relatively small area of about 20 square kilometres. It also scoured out small headwaters and pasture streams, causing slips which spread debris across roads.
Contractors were clearing the debris today.
“Most of the sediment appears to have come from streams that have been scoured out right up to their headwaters in pasture areas, where runoff is concentrated into small channels.”
A lot of topsoil and rich organic material had been lost, blocking culverts. Very little sediment appeared to have come from forested areas, he said.
“This is the sort of soil erosion and river management issues that Project Watershed seeks to address. It is clear from today’s flight that unprotected streams have suffered and are the main source of sediment being lost to the Waikato River.”
Environment Waikato had taken many calls from concerned residents – including Hamilton city water supply staff - who had never seen the river so brown.
Staff estimated that about 150 tonnes of mud an hour was flowing through the city, and the river did not appear to be clearing quickly.
Hamilton City Council staff are having no problems clearing the water through their usual filtration and floc treatments, and residents should not notice any change to their city supply.
Environment Waikato monitoring teams are keeping an eye on water quality over the next few days.
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