LAKES AT RQTORUA.

Auckland Star, Volume LXI, Issue 7, 9 January 1930, Page 5

LAKES AT RQTORUA.

TARAWERA AND ROTOMAHANA DANGER OF OVERFLOW ALLEGED. We have received from Mr. C. A. Whitney a long letter on the levels o£ Lakes Tarawera and Rotomahana and the possibility -of an overflow. Mr. Whitney says:— "It will not be long before Eotomahana Lake overflows, and when it docs some five miles by four miles of Water, with a depth of 150 feet, will rush into Tarawera Lake in a mighty torrent in the course of a few hours, until the surface of Rotomahana readies the same level as Tarawera. Before the Tarawera eruption Rotomahana Lake and the pink and white terraces were reached in canoes which paddled up the narrow stream between Tarawera and Totomahana* Lakes in a very short time, and there can be little doubt that the terraces were buried beneath the mud and pumice Which was thrown out of the old Rotomahana Lake and piled up between the two lakes, leaving a great dry crater instead of the lake. Soon water began to accumulate in the crater, until to-day a lake has been formed some five miles long and, four wide, and it must be about 500' feet deep, 350 of which are below the surface level of Tarawera Lake. /'The greatest recorded rise in.the lake since the eruption of 188G was 15 feet in six weeks, and when I was at Tarawera on December 19 last Rotomahana rose 2Jin in 24 hours. The rise of Tarawera Lake can easily be estimated wheu the dam -between the two lakes bursts, but an approximate estimate might put it at 20 to 24 feet higher than the present level of Tarawera Lake. This would cause a great flood down the Tarawera River, or, say, three times greater than the flood that took place when Tarawera Lake broke through the Bft embankment thrown up by the Tarawera eruption. If the great dam of Rotomahana, made of pumice and mud, breaks (thrQugh the' lake beginning to overflow) during the' night ■ when no one is prepared there would be an enormous loss of property and probably human lives, but on the other hand, if a-channel were deliberately made (at a very small cost) through the narrow ridge between the two lakes and opened on a given date early in the morning, eevryone would be prepared, and cattle, sheep, horses, etc., would be taken to pastures well above the low-lying areas in the Bay of Plenty. "To the west of Rotomahana Lake is another large lake, Rerewliakaitu. I have not been to this lake, but on the map it looks larger than Rotomahana. It is on a higher level than Rotomahana, and I am told it will not be long before it begins to overflow into Rotomahana. The public of Auckland know what' the Waikato River did the first few hours after it topped the spillway at Arapuni and how it moved and carried down the Waikato River millions of tons of pumice, etc. The country between Rotomahana and Rerewhakaitu is built up of pumice and sand (also between Tarawera and Rotomahana), which would be quickly washed away and a great river formed until both lakes reached the level of Tarawera. "There is nothing more sure than the fact that these two lakes.,must overflow sooner or later, and I strongly urge the authoritiesto "cut. a drain through the Rotomahana dam before it is too late, and before Rerewliakaitu overflows. Lowering Lake Rotomahana would in all probability uncover the terraces by washing the mud and pumice off them." We may add to Mr. Whitney's letter the following: — The difference in, levels between the two lakes, Tarawera and Rotomahana, according to the results of a topographical survey, is now 119 feet.- As for Rerewliakaitu, the figures show that it is actually lower to-day tlian. it was several years ago. The report states: "There is a gradual rise in the level of Lake Rotoinaiiana, which, if it continues, will cause the waters to overflow into Lake. Tarawera. In 1894 Tarawera was 55 feet below Rotomahana, and in 1924 the difference was 119 feet, a rise of 64 feet.in thirty years." The report comments on the fact that in the graph of variations of the height of Rotomahana, the oscillations could not be accounted for as the result of . meteorological changes, and were probably due to fluctuations in the thermal activity of * the region, "The contingency of .Rotomahana overflowing into Tarawera has not been overlooked, as you. can see from the Lands . and Survey Department's report," said an experienced surveyor, when the point' was mentioned to him. "Personally, I don't think it would make mucli difference even if Rotomahana did flow into Tarawera, which is. such a large body of water compared to Rotomahana. As. for Rerewliakaitu, the figures do not support the theory that it is rising."