Discharge of debris by Glaciar Hatunraju, Cordillera Blanca, Peru

The Editor,

Journal of Glaciology

SIR,

I have news about work at Laguna Paron from Ing° César Portocarrero, who has replaced Dr Benjamin Morales, and from Alcides Ames, who stayed for 6 months in Grenoble with Dr Louis Reynaud. The debouchment of the tunnel into this lake was completed at the end of 1983 by large-diameter boring instead of blasting as initially planned (Lliboutry and others, 1977, p. 252).

Prior to undertaking the engineering work, six corings were made under the direction of Ing° Andrés Huamán, which now allow further interpretation and discussion of the information given by Lliboutry (1977). The floor of the valley, where it dams the lake, was found to consist of strata of fine material, gravel, and “blocks” (cobbles or boulders). There is no correlation between the sequences in the different bore holes, 50 m apart from each other. One coring, 75 m deep, was done on Glaciar Hatunraju, at 4300 m a.s.l., where I had calculated that the glacier should reach its lowest altitude. Here, under 62 m of ice, there are 4 m of fine material, beneath which are 3 m of “blocks”, and 6 m of gravel. This substantiates the statement that the huge moraine creeps without any interstitial ice.

The glacier thickness at this site is less than predicted. In the published longitudinal section (Lliboutry, 1977, fig. 10, p. 265), the dashed line (corresponding to an annual negative balance b = 0.8 m/year) should be adopted and, between the calculated positions of the bottom under points (41) and (38), it should be replaced by a straight line. With b = 0.8, the discharge of Glaciar Hatunraju across its upper cross-section should be 110000 m³ of ice per year.

Cores showed a mixture of ice (60% by weight), quartzose sand (25%), and pebbles 1–3 cm in size (15%). By volume, these figures become 81.8, 11.4, and 6.8%, respectively. This finding contradicts the statement (Lliboutry, 1977, p. 257) that a small ice cliff showed no morainic inclusions. Nevertheless, this exposure was in the upper part of the covered glacier; the ice found at the surface there was formed in the lower part of the accumulation zone, which is probably not reached by debris falling from the surrounding rock walls.

Thus, my previous estimate of the amount of debris carried by the glacier has to be revised. At point (38), the discharge is 24800 m³ of ice per year, including 18.2% of debris, i.e. 4520 m³ per year. The thickness of the ablation moraine there can be estimated at 1.2 m (corresponding to the ablation of debris-laden ice over 80 years, on the 600 m up-stream). Thus, about 560 m³/year of ablation moraine has to be added. The discharge of debris into the valley should be about 5080 m³/year, instead of 3000 m³/year. The Hatunraju moraine (92 × 10⁶ m³) could have been built up in 18000 years under the present conditions, without assuming that there is soft material beneath the glacier in its upper part, and that this is carried into the valley. If the latter assumption is made, the time necessary might be halved.

A tentative estimate of the amount of cirque erosion during the Holocene can be inferred. The rock wall with hanging patches of ice, which surrounds the accumulation zone of Glaciar Hatunraju, is about 2700 m long and 800 m high. To deliver 5080 m³ of debris per year, it has to recede, due to frost shattering, at a mean rate of 2.4 mm/year, which is only 24 m during 10000 years of the Holocene. Although this estimate is conservative, it shows that it took the whole of the Pleistocene to form the cirque.