51. ORANGE RIVER NAMA KAROO

Synonym:

Orange River Broken Veld (A32).

Statistics:

53 708 km²; area transformed unknown; 1.47% conserved.

Locality & Physical Geography:

As its name suggests, this region occurs within the hot, arid drainage basin of the Orange River, extending westwards as far as Vioolsdrif. Altitude ranges from 350 m in the mountains in the east to about 250 m near Vioolsdrif in the west.

Climate:

Rainfall (150 to 350 mm per year) is low, occurring in the late summer in the east and in the late autumn in the west.

Geology & Soil:

Orange River Nama Karoo occurs on soils derived from the ancient basement granites and gneisses of the Namaqualand Mobile Belt on the south edge of the Richtersveld Craton. Red and yellow apedal, freely draining, young soils dominate most of the area. Deep alluvial soils occur along the Orange River.

Vegetation:

In places, the region is very rocky and possesses a "broken" topography with Quiver Tree Aloe dichotoma, Bushman Poison Tree Euphorbia avasmontana and Aggenys Milkbush E. gregaria normally associated with the steep slopes of the mountains and hills of the area. On the pediments, Spike-flowered Black Thorn Acacia mellifera, Threethorn Rhigozum trichotomum, Shepherd's Tree Boscia albitrunca and Stink Shepherd's Tree B. foetida are common trees and shrubs, while Silky Bushman Grass Stipagrostis uniplumis often dominates the plains, especially after good summer rains. There are abundant thickets along the banks of the Orange River itself, with Wild Tamarisk Tamarix usneoides, Buffalo Thorn Ziziphus mucronata and Camel Thorn Acacia erioloba common along the dry river beds of the tributaries as well.

Key Environmental Parameters:

The complex geology and broken, rocky terrain result in a large number of distinct vegetation communities within Orange River Nama Karoo. This type tends to occur on the granite-derived soils rather than the shale-derived, clayey soils of the other Nama Karoo types. Nothing is known of the role wild animals, especially nomadic species, played in the ecological functioning of this vegetation type.

Economic Uses:

Along the Orange River the water and deeper alluvial soils have enabled extensive crop irrigation, the most important being - cotton, lucerne, table grapes and sultanas. Elsewhere, the lack of irrigation restricts farming to small stock, mainly Dorper Sheep and Boer Goats. Mining is prevalent in the area.

Conservation Status:

The Augrabies Fails National Park is the largest conservation area within Orange River Nama Karoo. Although only

a small portion of the area is conserved, careful farming should preserve most of the plant species. Mesquite Prosopis glandulosa and Threethorn Rhigozum trichotomum, are aggressive invaders of riverine areas, mainly in parts which are heavily grazed.

Key Reference:

Acocks (1988)

Author:

Timm Hoffman.