Sierra Leone - State of the Marine Environment 2015

the Galinas delta. The St. Ann shoals trends northwest from Sherbro Island, reaching the outer shelf at the southern edge of the areas. This shoal is roughly 30 kilometre wide and rises to depths of 5–14 meter and the surface is marked by several linear sand ridges oriented northeast southwest which are 3–5 meter wide and up to 7 meter high. 2.5. Major river deltas/wetlands The Sierra Leone coastal area can be divided into four main hydrological areas. These are the Scarcies River, Sierra Leone River, Sherbro River and the Gallinas and Mano Rivers basins (Fig. 2.3). 2.5.1. Scarcies River basin The river is tidal and during the rainy season rises about 2.7m. The wide estuary mouth has mud banks and sand bars forming Yelibuya and Kortimaw islands. Further inland, it splits into the Great and Little Scarcies Rivers which are relatively narrow and lined with mangroves. 2.5.2. Sierra Leone River basin The main rivers entering this hydrological area are the Rokel, Port Loko creek and Kumrabai Creek. 2.5.3. Sherbro River basin Three major river systems, the Taia, Sewa and Wange rivers enter the Sherbro River Estuary through a complex system of brackish water channels draining an extensive area

an indigenous traditional way of preserving fish caught for sale, and also as an important source of fuel wood (Chong, 1987). The environmental role of this natural resource includes, coastal barriers in storm protection, flood and erosion control, and as habitat nursery ground for fish, shrimps and other marine fauna. 2.4. Structural systems 2.4.1. Canyons and Shelf break According to surveys conducted by a number of international institutions, the Sierra Leone continental margin is incised with a number of canyons which serve as sediment traps (Fig. 2.2). The outer shelf lies below 60–70 meter depth and is smaller in width with greater angles of inclination of the bottom. In some parts bedrock is common. This part of the shelf is commonly incised by the heads of canyons. The state of these canyons is stable. 2.4.2. Seamounts Seamounts are not characteristic of Sierra Leonean waters. 2.4.3. Large gulfs Large gulfs are also not characteristic of Sierra Leonean coastal and marine areas. 2.4.4. Offshore banks, shoals, islands Amongst the prominent geomorphic features in the southern part of the shelf are the St. Ann shoals and

Makeni

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S i e r r a i r r L e o n e

Freetown

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Kenema

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Monrovia

Legend

country borders rise escarpment canyon seamount shelf valley fan/apron terrace Slope Shelf - high profile Shelf - medium profile Shelf - low profile Abyss - mountains Abyss - hills Abyss - plains

Figure 2.2: Geomorphology of the Sierra Leone maritime zone (GRID-Arendal, Harris et al. 2014)

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