The crater lake (on the left) has just about reached the top of the dam, and is still seeping through. Photo / DoC
The rate the crater lake on Mt Ruapehu is rising has slowed, as seepage out of the lake's tephra dam increases.
Department of Conservation workers inspecting the lake on Thursday found nothing untoward, though they maintain the earliest a lahar could occur is February or March.
The lake
level had risen slightly from last week to be 2.7m from the top of the dam, which blocks the lake's usual outlet.
DoC scientist Harry Keys said the lake level indicated that its rising rate had slowed considerably.
The cold snap over Christmas had slowed the process of snow melting, he said.
The dam seepage, which had been increasing the erosion on the dam, had increased from 10 litres a second to between 10 and 20 litres a second.
"If it continues to increase, as we can reasonably expect, it may soon start to rival inflows during cooler and dry-windy periods," Dr Keys said.
"In that case the lake would start to drop or level off rather than rise. The lake may also be starting to warm again which will increase the likelihood of evaporative losses, further limiting the rate of lake level rise."
Dr Keys says it is only a matter of time until the lahar sends up to one million cubic metres of water - enough to fill 400 Olympic-sized swimming pools - down the Whangaehu Valley.
Safety measures, including a barrier of boulders and gravel 300m long, 20m wide and up to 4.6m high, are in place to divert the lahar into the Whangaehu River, away from any residential areas.
During the visit, workers surveyed the crater lake for any changes in shape, which could indicate movement in the volcano chamber or water pressure from a refilling lake.
DoC workers accidentally set off the lahar alarm system, and were thankful the systems in place quickly alerted police that it was a false alarm.
