Description

Lake Otjikoto lies some 20 km W-NW of Tsumeb along the main road to Ondangwa and Namutoni. „Otjikoto“ is the Herero word for „deep water“, and is in fact a collapsed sinkhole in dolomites of the Neoproterozoic Maieberg Formation of the Damara Sequence (Hedberg, 1979). It’s location  possibly has some structural control, as a magnetic anomaly passes adjacent to the lake. Lake Guinas is also located in bedded dolomites with associated structural control. It is larger than Lake Otjikoto, having a maximum diameter of 140 m and a depth of 153 m (Penney et al., 1988). The waters of Lake Otjikoto and Lake Guinas are utilised for irrigation (Hoad, 1992; 1993).                  .

Sinkholes commonly occur in areas underlain by carbonate rocks, and form through dissolution of the carbonates by meteoric waters. Such dissolution often starts from underground, and the roof of the structure eventually collapses, leaving behind a circular depression. The Karstveld to the northwest of Tsumeb shows a number of sinkholes and may be traced from the Tsumeb-Grootfontein-Otavi Triangle westward through the Etosha National Park and from there northwards. Cavities in dolomitic rocks form when water percolates through fractures and carbonic acid in the water dissolves the rock. Gradually these cavities grow into caves that usually contain water. In some cases the roof of the caves collapses to form open sinkholes, such as Otjikoto and Guinas. The lakes at Otijkoto and Guinas represent "windows" to the surface of the groundwater flow from the Otavi Mountain Land towards the N.

The depth of the Otjikoto sinkhole possibly exceeds 75 m. It has a diameter at surface of about 100 m surrounded by vertical cliffs, but becomes wider at depth (Fig. 8.17.1). The storage of the lake is of the order of 400 000 m³, and the Government permitted maximum abstraction for irrigation purposes amounts to 1.3 million m³ per year (Hoad, 1992; 1993). In the period 1973 to 1980 the lake water level actually rose by 12 m reflecting groundwater recharge due to a number of above mean annual rainfall years. Since 1980, however, the water level has been almost continuously declining, as most of the annual rainfall has been below average and groundwater recharge extremely poor. This drought scenario was observed in the entire Otavi Mountain Land and its surroundings (Schmidt & Plöthner, 1999).

The water of Lake Otjikoto can be characterised as a magnesium-calcium-bicarbonate water, which has a low mineralisation of 470 mg/l, a pH value which varies between 7.0 and 8.7 in different parts of the lake and a temperature of 27 °C (Marchant, 1980). The predominance of magnesium over calcium is typical for dolomite groundwaters.

Results of stable isotope determinations suggest that the water of Lake Otjikoto and Lake Guinas is being recharged from an area in the Otavi Mountain Land at an average altitude ranging from about 1600 to 1900 m above sea level. Such altitudes occur in the area S of Tsumeb and N of Kombat. Radiocarbon dating of the dissolved inorganic carbon of the lake water by Vogel (1978) resulted in¹⁴C water age ranges from 600 to 1600 years before present for Otjikoto and Guinas, respectively. Thus, the travel rate of groundwater from the mountainous recharge areas to the lakes, amounts to 30 to 40 m/year. This is a rate typical for groundwater circulating in fractured, but not highly karstified aquifers such as the prevailing dolomites of the Otavi Mountain Land and its surroundings (Ploethner et al., 1997)

A 1.3 m core of sediments from a sloping platform at about 52 m depth in the central part of Otjikoto Lake was obtained in 1990. The core consists of a grey homogeneous fine silt with small white and dark specks and vague white laminations between 44 and 65 cm. Attempts were made to date the core with radiocarbon. Conventional ¹⁴C ages of 1300 and 3200 years BP at a depth of 20 and 94 cm, respectively, yield a sedimentation rate of approximately 0.5 mm/year (Scott et al., 1991). On the basis of pollen data the Otjikoto sequence indicates that during Late Holocene times, some 3500 years ago, a relatively dry climate prevailed, which gradually changed into a temporarily wetter phase probably about 1000 years ago, which is in good agreement with the palaeoclimatic concept of Heine (1992).

Brief History

The water level was first measured by Sir Francis Galton in 1851, and Galton was also the first to record the presence of fish, which were later to be found new to science, in Lake Otjikoto (Galton, 1889). In 1933, the English scientist and explorer Dr. Karl Jordan collected fish specimens from the Otjikoto and Guinas lakes and made them available for formal scientific description (Trewavas, 1936), including two native species, Tilapia guinasana and the southern mouthbrooder P.philander (Skelton, 1990). Many changes have taken place since Galton and Jordan visited the two lakes. A number of man-made structures have been built including platforms for large pumps and water gauging installations. It is also well known that the retreating German forces during World War I dumped large quantities of weapons and ammunition into Lake Otjikoto in 1915. A large portion of that military equipment was recovered by the South African troops and later used in East Africa (Penrith, 1978). Early in the development of mining activities at Tsumeb, water required for processing was transported by ox-wagons from Lake Otjikoto to Tsumeb. In 1907, a high pressure pipeline was laid and the steam engine built by M. Neuhaus & Co., Luckenwalde to run the pumps for conveying 500 m3/day of water from Lake Otjikoto to Tsumeb can still be seen on the southern side of the lake. In 1909, excessive rainfall resulted in a rise of lake water of 9 m in Otjikoto and some 30 m in Guinas in 1909 (Jaeger & Waibel, 1920). As a consequence, the Otjikoto pump scheme was inundated and Tsumeb had to be supplied with water carried by railway from Otavi (Gebhard, 1991).