botswana/namibia - Cour international de Justice

382. When a river channel bifurcates, the "main" channel will attain a local grade profile that
will enable it to convey its share of the bifurcated flow, which is larger than the portion of the
flow in any other channel which is part of the bifurcation.
(i) Depth is the criterion of navigability
383. As reported in the Botswana Memorial, the 1985 Joint Survey established the greater
depth of the northern channel: "Once the river has cleared the small sand bar to the north of
the National Park HQ, the cross-sections show a steady depth of 5 to 7 metres, deepening to
10m. after the entry of the backwater stream at Kabuta, and to 13 m. at the juncture of the
eastern section into the main stream" (Botswana Memorial, p.94, para. 221). Whereas "as
regards the south channel, whilst there is a depth of some 4 m. average of water in the eastern
section of this channel, both the eastern and western entrances to the southern section of this
channel are barely more than 2.5 m. in depth with a thick growth of reeds extending from
each side, with a deeper 'lagoon' area of 6 m. in the centre of this southern section" (para.220).
The growth of reeds across the western entrance of the southern channel is reflected in the
photograph taken in September 1997 (at page 153).
384. The Joint Survey Team included three senior Hydrologists from the respective Water
Affairs Departments of South Africa and Botswana, as well as a Surveyor-General, a Director
of Surveys and Lands, a Deputy Director-General of Surveys and an Assistant Director of
Surveys and Lands. The positions were therefore plotted by highly qualified and experienced
land-surveyors and evaluated by eminently qualified hydrologists. No calculations were
necessary as the team was using modern self-recording depth gauges which gave a real-time
print-out of the cross-sections with depth plotted graphically against a horizontal scale.
(ii) The criterion of capacity to carry larger flow
385. The main channel is the channel which carries the greater amount of water. The
discharge of any conduit is proportional to the water velocity and the cross-sectional area,
being the product of the width and area of the channel. Whether the flow is high or low the
velocity will be determined by depth, width, slope and hydraulic radius (see the diagrammatic
illustration of a channel cross-section at page 154). It is not correct that a greater flow in time
of flood will increase the velocity. The gradient of the river contributes to the velocity of the
channel since it causes and accelerates the flow.
386. The main channel will have its width, depth, and velocity compensatorily adjusted to
conveying its share of the bedload-flow and sediments. This compensatory adjustment arises
out of the relationship that width, depth and velocity have to discharge. As stated in Leopold.
Wolman and Miller (1964) p.215, "With increasing discharge at a given cross-section, the
width, mean depth, and mean velocity each increase as power functions..." (see Professor
Sefe's Second Opinion, Appendix 5). Thus identification of the main channel of the Chobe
River can only be properly done by considering all the hydraulic parameters as well as the
volume of flow.
387. To sum up as regards criteria to establish the main channel, it has been shown from the
1985 Joint Survey Report and the measurement of flow at the Chobe Gauging Stations that
the northern channel satisfies both the criteria of greater depth and greater flow. From the
analysis of the aerial photographs and satellite imageries clear evidence of the greater width
and depth of the northern channel compared to the southern is provided. The itinerary of the