PLAN TO LOWER LAKE LEVEL AT ROTORUA

Press, Volume XCVII, Issue 28754, 27 November 1958, Page 21

PLAN TO LOWER LAKE LEVEL AT ROTORUA

“The Press” Special Service

ROTORUA, November 26. Discussions will begin in Rotorua soon on a proposal to lower the level of lake Rotorua by as much as 18 inches by dredging the bar across the Ohau Channel, which connects the lake with Lake Rotoiti.

The chairman of the borough council works committee, Mr J. B. Wilson, regards a lowering of the lake level as the only permanent solution to recurring floods in the Utuhina stream. The borough engineer, Mr D. C. A. List, supports him.

The Utuhina and its tributaries drain a large area of hill country west of Rotorua It empties into the lake at Ohinemutu.

With the gradual loss of natural bush in the region of its headwaters the stream carries an increasingly large volume of water after heavy rain.

Mr Wilson believes that the presence of a bar at the mouth of the stream, and the high level of Lake Rotorua, cause this water to bank up and flood the low-lying land on its lower reaches.

Control of Flow The level of the lake is controlled by the flow of water through the Ohau Channel which falls about two feet in a mile.

On paper the plan is simple enough. By dredging the channel more water would flow from Lake Rotorua into Rotoiti. This would cause a fall in the level of Lake Rotorua and increase the rate of flow down the lower reaches of the Utuhina, thus eliminating flooding.

The extra flow through the Ohau Channel, however, would cause a rise in the level of Rotoiti.

Dredging the Ohau Channel, says Mr Wilson, would make it possible for cruise launches to operate between the two lakes for the first time for years.

The project to dredge the channel is viewed less enthusiastically by trout fishermen. Recent research has shown that numbers of Rotoiti trout use the channel as an access to Lake Rotorua and the spawning streams flowing into the lake from the west. If the flow of the channel were “killed" it might well affect this movement.

The north-western area of Lake Rotorua is characterised by a shallow shelf extending well out from the shore and normally covered by about four feet of water. A permanent fall of 18 inches would almost halve the depth and could create a difficult navigation problem for large launches. It could also lay bare fairly large areas of beach.

Effect on Springs According to Mr J. Healy. Government volcanologist at Rotorua, a fall in the lake level would eventually cause a corresponding fall in the level of “permanent ground water." This would mean that some springs would give a reduced flow of water and others could even disappear.

"It is difficult to forecast how this would affect Fairy. Rainbow and Hamurana Springs.” said Mr Healy, “as all these derive much of their water from subsurface drainage from the upland country. “However, it could possibly affect thermal activity. Whether it would affect the geysers would depend largely on how much the

level was dropped. A fall of 18 inches might be negligible.”

Mr Healy said that observations made by the Department of Scientific and Industrial Research revealed that, over a period of years, the lake levels could differ by as much as 4J feet. The Jevel varied about 18 inches with the seasons each year. The lake level is not likely to be lowered without further consultations with the Marine Department, the Ministry of Works and the Electricity Department. . The last two will be vitally interested because of their longterm proposals to harness the Kaituna river to generate power. The Kaituna has its source in Lake Rotoiti.