Civil Defence says the risk of an overflow on the Hapuku River near Kaikoura has dropped, but there is still a dangerous backup of earthquake debris on the river.
The newly created dam - a 150-metre-high wall of mountain debris from this week's earth - is one of dozens known to have formed by slips in the district's waterways, caused by Monday's 7.8 magnitude earthquake,
Earlier, Canterbury Civil Defence said the lake now forming on the Hapuku could rapidly overflow because of heavy rain.
It now says the current and forecast rainfall is nowhere near enough to fill the lake.
However, it said the dam's stability could not be confirmed, because helicopter access was restricted by weather yesterday, and the risk of aftershocks remains.
The Department of Conservation's Puhi Puhi camping ground in the river valley was evacuated yesterday because of the downstream risk.
GNS Science says all rivers from the Awatere to the Hurunui could contain landslide dams and should be considered high risk areas.
Marlborough's Civil Defence controller Richard McNamara said it could be years before north Canterbury recovered from the earthquakes.
He says Monday's quakes triggered up to 60,000 slides and some have caused major blockages, on the Ure River south of Ward, and on the Hapuku River north of Kaikoura.
Residents at risk have been evacuated, but rain overnight has put further pressure on the banks, threatening farms, houses, and bridges when they burst.
Mr McNamara told Morning Report the force of the earthquakes had caused colossal damage.
Mr McNamara said some homes that have just been rebuilt after the Seddon earthquakes have been destroyed.