Lake Rotomahana

Sun (Auckland), Volume III, Issue 898, 15 February 1930, Page 6

Lake Rotomahana

NO DANGER OF FLOODIRq Engineer’s Opinion ‘EXCEPT for the possibility of a cataclysmic earth mov L ment in the vicinity, there is no immediate risk of Rot*' mahana lake bursting the barrier separating it from L*i Tarawera and flooding the Rangitaiki Plains.’’ This stat * ment was made to The Sun by Mr. R. G. Macmorran. Chi *4 Drainage Engineer to the Government Land Drainage dT partment, who lias a lengthy acquaintance with the lafc. country.

In fact, the department has kept careful record of the rise and fall of the surface of Lake Rotomohana for the past five years and the monthly measurements, as shown on the graph herewith, indicate that the lake exhibits seasonal movements, but that the net average annual gain in height is two feet. As the highest point of the 700 ft thick barrier is 30 feet above the present water level, only an unforeseen event could open a way from one lake to the other. If danger really threatened the department could provide a runoff capable of holding the higher lake at a determined level. The run-off would be the amount by which the inflow to the lake exceeds the loss byleakage and evaporation. Prior to the eruption of Tarawera, Lake Rotomahana was 10 acres In extent and was then 40 feet higher than Tarawera Lake, being drained into Tarawera by the Kaiwaka Stream. Adjoining it was Lake Rotomakiri. "With several tiny ponds they acted as gathering places for the drainage into the very big depression that is now comprised in the boundary of Rotomahana. After the eruption, which might have been termed Rotomahana eruption, as the actual burst was at the junction of the Rotomahana crater and Tarawera Mountain and most of

the subsequent activity was in Rotomahana depression, Lake Rotomahana shrunk in size and the Kaiwaka stream bed was filled with mud, pumic and volcanic waste. It is this barrier in the old stream valley that now holds back the waters, and at the level of Lake Rotomahana it is 700 feet thick. Rotomahana’s surface has risen 90 feet (approx.) since 1883. and not 300 feet, as has been stated. UNCOVERING THE TERRACES In 1906 the lake was approximately 100 ft above Tarawera; in 1925 t was 118 feet and it is now 128 feet, approximately, with an area of 10 square miles. The rainfall for the 36 square miles of watershed that normally drains into the lakes is not known, but the graph reveals that the lake rises to a maximum about December and recedes until May. During the last seven years there has been abnormal rainfall over the Auckland Province and the peak rise in the lake was in 1926. The movements of the surfaces of other lakes is interesting, too. Lakes Rotoiti and Rotoehu have risen and fallen in recent years and Lake Tikitapu is now many feet lower

I than 20 years ago. If there series of reduced rainfalls t, i V! j bable that the level of Rotom.T ’ will recede owing to the the volume of drainage rearhi. ’’The drainage of RotomawL I I Tarawera would be child’s Ui I | some of the jobs that the denar?; has carried out,” remarked Mr v j morran, "and so far as the Rair I Plains is concerned the onb * I guard is that when the ti me and that may not be in my 57* I career, stability in the level of P mahana can be artificially ar tained.” ’ There is a sentimental lon s i n . by many folk to see Lake rV mahana drained, since benu?? the milky waters of the lake* New Zealand's lost gloriet ?£ i superbly beautiful and vwmj.' fully wrought White and PinkTe? races. Overlain by an unknown thick* of Tarawera ash, and the scenm? tion of 44 years silting of the to and submerged by many feet of are the terraces—or their wreck. \ Percy Smith, of the Lands and Sum Department, who was sent to reao on the Tarawera eruption withia few days of its happening, beUntthat these masterpieces of nsta-i w?re ruined. Since then a legend h

grown up that if the waters *,:•§ rolled away and the ash and t a cleaned off, the terraces would { restored in their pristine glory g:, be the most admired spot in oi l wonderful land. Many a visitor has been thrilled t the vision of uncovered, terrace*,, c the guides have pointed out the WE [ steaming site of the drowned pit beauty, and sadly indicated a poir in the lake as the whereabouts of pearly white terraces. The wealth to be obtained by ruing sunken Spanish galleons e. , wrecked treasure ships would be t finitesimal to that accruing to St« Zealand if only the terraces could; recovered. Hundreds of tourists woi. flock to the region. Asked It i opinion on the recovery of the tr races, Mr. Macmorran replied enigti ically, “Perhaps.” He added th« only way to settle the problem wot : ; be to take borings, as was done determine the nature of the bed ! the Waitemata. Lowering the Bat mahana Lake level to that of Tk wera would still leave about 400 l* of water covering the lowest port* of the old Rotomahana crattr.