Note: Although the Sierra Leone River can be flown over by air, or driven around by automobile, this earthcache requires you to sail across the water by ferry.
Welcome to Sierra Leone!
As you sail from the airport at Lungi to the capital of Freetown, have a look at the water around you. While by now you may feel like you’re on the open ocean, you are in fact at the edge of an enormous estuary – the largest in Africa – at the mouth of the Sierra Leone River.
Estuaries are aquatic environments that form at the mouths of coastal rivers. Marine influences like tides, waves, and saline water combine with flows of fresh water and sediment to create a “transition zone” between river and ocean environments.
The Sierra Leone River Estuary is bounded on the north by a coastal plain indented by creeks, and on the south by a mountainous peninsula with peaks ranging from 330-950 m in height. It extends 27 miles from its mouth to Tumbu Island, beyond which the Port Loko Creek joins the Rokel River. From Tumbu Island southwestwards, the estuary assumes a width of 2.5 miles until it reaches Targrin Point (the location of the ferry terminal), where it turns northwestwards and widens gradually to about 7 miles before reaching the Atlantic Ocean.
Most of the estuary is shallow, except for a narrow, moderately deep channel that follows its course until, off Targrin Point, it becomes nearly parallel to the northern shores of the peninsula. The channel varies in depth from 7 m off Pepel, to about 33 m opposite Cline Bay (at Kissy) through to its mouth in a downstream direction. On either side of the channel the depth of water is considerably less than 10 m and in the upper reaches extensive mud flats are exposed during low tide.
There is no sand bar at the mouth of the estuary where the deep channel joins the Atlantic Ocean off Cape Sierra Leone. However, the deposition of sand, especially on the northern side of the estuary, results in the formation of a relatively broad sand bank, which becomes exposed at low water springs. This sand bank reduces the depth of the mouth of the estuary, but does not seem to reduce the amount of water entering or leaving it, as the strength of the tidal stream is often very great and keeps the channel clear.
One way that estuaries are classified is based on their geomorphology. Four common classifications are: drowned river valleys, lagoon-type or bar-built, fjord-type, and tectonically produced.
Drowned river valleys were typically formed between about 15,000 and 6000 years ago following the end of the Wisconsin (or 'Devensian') glaciation when a eustatic rise in sea level of 100 to 130 m (330 to 430 ft) flooded river valleys that were cut into the landscape when sea level was lower, creating the estuarine systems. Additionally, the general subsidence of coastal regions contributed to the development. Well-developed drowned river valleys are generally found on coastlines with low, wide coastal plains. Their width-to-depth ratio is typically large, appearing wedge-shaped in the inner part and broadening and deepening seaward. Water depths rarely exceed 30 m (98 ft).
Lagoon-type or bar-built estuaries are semi-isolated from ocean waters by barrier beaches (barrier islands and barrier spits). Formation of barrier beaches partially encloses the estuary with only narrow inlets allowing contact with the ocean waters. Bar-built estuaries typically develop on gently sloping plains located along tectonically stable edges of continents and marginal sea coasts. The barrier beaches that enclose bar-built estuaries have been developed in several ways: 1) upbuilding of offshore bars from wave action, in which sand from the seafloor is deposited in elongate bars parallel to the shoreline, 2) reworking of sediment discharge from rivers by wave, current, and wind action into beaches, overwash flats, and dunes, 3) engulfment of mainland beach ridges (ridges developed from the erosion of coastal plain sediments approximately 5,000 years ago) due to sea level rise and resulting in the breaching of the ridges and flooding of the coastal lowlands, forming shallow lagoons, and 4) elongation of barrier spits from the erosion of headlands, with the spit growth occurring in the direction of the littoral drift due to the action of longshore currents. Barrier beaches form in shallow water and are generally parallel to the shoreline, resulting in long, narrow estuaries. The average water depth is usually less than 5 m (16 ft), and rarely exceed 10 m (33 ft).
Fjord-type estuaries are formed in deeply eroded valleys formed by glaciers. These U-shaped estuaries typically have steep sides, rock bottoms, and underwater sills contoured by glacial movement. The shallowest area of the estuary occurs at the mouth, where terminal glacial deposits or rock bars form sills that restrict water flow. In the upper reaches of the estuary, the depth can exceed 300 m (980 ft). The width-to-depth ratio is generally small. When estuaries contain very shallow sills, tidal oscillations only affect near surface waters to sill depth, and waters below sill depth may remain stagnant for very long periods of time, resulting in only an occasional exchange of the deep water of the estuary with the ocean. If the sill depth is deep, water circulation is less restricted and a slow, but steady exchange of water from the estuary and the ocean occur.
Tectonically produced estuaries are formed by subsidence or land cut off from the ocean by land movement associated with faulting, volcanoes, and landslides. Inundation from eustatic sea level rise during the Holocene Epoch has also contributed to the formation of these estuaries. There are only a small number of tectonically produced estuaries.
The Sierra Leone River has borne witness to a dark period of human history. It is from these shores that thousands of Africans were enslaved and shipped overseas as part of the European slave trade in the 17th and 18th centuries. Bunce Island, a small island in the eastern part of the estuary, was the last African land that many slaves would ever set foot on. The Gullah people in Georgia and South Carolina (United States) are direct descendants of these slaves. When in Freetown, take the time to visit the Sierra Leone National Museum (a minute’s walk from the Cotton Tree in downtown Freetown) to learn much more about this history.
Sources:
- Findlay, Ivan W.O., Marine Biology of the Sierra Leone River Estuary, 1978.
- http://en.wikipedia.org/wiki/Estuary
- http://en.wikipedia.org/wiki/Sierra_Leone_River
- http://en.wikipedia.org/wiki/Bunce_Island
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To log this earthcache, e-mail the cache owner your answers to the following questions. Some questions can be answered by observing your surroundings; others, by consulting the sources listed above. Post the picture and story (#5 and #6) with your log. DO NOT post answers to the first four questions in your log.
1. Based on the information above, what type of estuary would you say the Sierra Leone River is?
2. What is the approximate length and width of the estuary?
3. What two streams to the east join to form the Sierra Leone River?
4. Estimate the time it took for the ferry to cross from Lungi to Freetown (or vice versa).
5. (Optional) Post a photo of yourself (optional: holding your GPS) crossing the river on the ferry. (If crossing at night, try to get the lights of Freetown in the background or, failing that, demonstrate somehow that you are on the ferry.)
6. (Optional) What is the purpose of your travel to Sierra Leone?
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