ROTOMAHANA'S LEVEL.

Auckland Star, Volume LXI, Issue 33, 8 February 1930, Page 12

ROTOMAHANA'S LEVEL.

A RISE IN THE LAKE SURFACE. POSSIBILITIES OF AN OVERFLOW. Mr. C. A. Whitney, writing on- th© subject of Lake Rotomahana overflow, says:— I am indebted to Mr. Warbrick, of Wairoa, for further information re the levels of Rotomahana Lake. On January 3, the lake surface was 1104 ft 10in above sea level; between January 8 and January 18 it rose s§in, and since then there has been another rapid rise. The depth of the lake at the present time is 800 ft, of which 146 ft is above the level of Tarawera Lake. The height of the pumice and mud wall, is about 1 lift from the lake on a very steep grade; from top of pumice and mud wall to Tarawera Lake it falls away very quickly in a gradual slope for about one mile, and water travelling over these 14Gt't of down grade (or 102 ft, as it will be when it commences to overflow) the speed and force of the water will be terrific; (the start of the Waikato over the spillway at Arapuni will Jra child's play to it and nothing can #9fcp it) and as I said in my letter, a few hours will reduce Rotomahana by 120 ft, or more, and Tarawera outlet can't possibly draw off the water as fast as it enters the lake, which is most fortunate for the settlers in the plain, but it is not a difficult calculation to ascertain what the rise of Tarawera would be whilst great quantities of water were being drawn off by the river. The object lesson is in the boulder jamb in the valley or gorge, H miles down the river; starting at say lift and tapering nearly two miles to the fall of 100 ft, a boulder-filled valley—whilst boulders were rolled down the river when a similar flood took place, that is, the bursting of a pumice dam between Rotomahana and Tarawera. Mr. Warbriek and Mr. Ilill made a •thorough examination of Lake Rerewhakaitu on January 14, and they tell me there is no fear of it ever tipping" the ridge between it and Rotomahana and that in all probability it will overflow iu an easterly direction into the Rangitaiki, but the possibility of the water finding an underground way to Rotomahana must not be forgotten, more especially as Mr. Brett, in his letter, says the ridge between Rotomahana and Rerewliakaitu is mud and that would wash away quite as quickly as pumice or sand. At the present four years' recorded rise ill the lake, it will be over the top in three or four years, probably less. It would be interesting to know if anyone now living knew where the outlet of Rerewliakaitu was before the eruption in 18SG, when the barrier between the lakes was raised either by the enormous fall of mud or by the upheaval of the earth at that point. On my return from Scotland at the end of October next I shall make a point of inspecting the rise in Rotomahana between now and then. It has been suggested to draw off the water by syphon, but it does not appear to me to be practicable. Moreover, it would not eliminate the danger, as any day the pressure of water in the lake may find a seep in the barrier between the two lakes, which would quicklv : enlarge and cause the whole dam to ■burst. Surveys should be made to ascertain whether Okataina Lake surface is below the level of Rotomahana Lake surface. It is very high just now and rising still.