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20-402: Lake Itasy has an area of 3,500 hectares. It is the fourth largest lake on Madagascar, after lakes Alaotra , Kinkony , and Ihotry . The lake has an average depth of 4 meters, and reaches up to 10 meters deep. The water level varies seasonally. There are marshy areas in the southern and eastern parts of the lake. Lake Itasy lies in the volcanic Itasy Massif. The volcanic activity in the region

40-434: A range of habitats, including open water, reedbeds, marshes, and rice paddies. The lake itself covers 900 km (350 sq mi). Lake Alaotra was declared a wetland of international importance under the international Ramsar Convention on February 2, 2003. The longfin tilapia ( Oreochromis macrochir ) was introduced into Lake Alaotra from the mainland in 1954 and proliferated quickly. By 1957, it provided 46% of

60-413: A sufficiently strong force to produce significant sedimentation. Settling is the process by which particulates move towards the bottom of a liquid and form a sediment . Particles that experience a force, either due to gravity or due to centrifugal motion will tend to move in a uniform manner in the direction exerted by that force. For gravity settling, this means that the particles will tend to fall to

80-480: Is called aggradation . The rate of sedimentation is the thickness of sediment accumulated per unit time. For suspended load, this can be expressed mathematically by the Exner equation . Rates of sedimentation vary from less than 3 millimeters (0.12 in) per thousand years for pelagic sediment to several meters per thousand years in portions of major river deltas . However, long-term accumulation of sediments

100-559: Is designated an Important Bird Area . The environment of the lake and its tributary rivers have been altered by pollution, erosion and siltation, habitat destruction, and the introduction of exotic species . The native fish Ptychochromoides itasy , last observed in the lake in the 1970s, was thought extinct until a population was discovered in the Sakay River. Lake Alaotra Lake Alaotra ( Malagasy : farihin' Alaotra , [faˈrihin ˈaloʈʂə̥] ; French : Lac Alaotra )

120-461: Is determined less by rate of sedimentation than by rate of subsidence, which creates accommodation space for sediments to accumulate over geological time scales. Most sedimentation in the geologic record occurred in relative brief depositional episodes separated by long intervals of nondeposition or even erosion. In estuarine environments, settling can be influenced by the presence or absence of vegetation. Trees such as mangroves are crucial to

140-491: Is opposed by the diffusion of the particles. The distribution of sediment near the boundary comes into sedimentation equilibrium . Measurements of the distribution yields information on the nature of the particles. In geology , the term sedimentation is broadly applied to the entire range of processes that result in the formation of sedimentary rock, from initial formation of sediments by erosion of particles from rock outcrops, through sediment transport and settling, to

160-424: Is relatively recent, and lake was formed when lava flows blocked a river, forming a lake behind it. The landscape around the lake is covered with Strombolian scoria cones and trachyte domes. The volcanic landscape around the lake has many smaller lakes ( maars ) and peat bogs. The Andranomena and Mariandrano rivers empty into the lake from the east. The Lily River is the lake's outflow, draining westwards from

180-730: Is the deposition of sediments which results in the formation of sedimentary rock . The term is broadly applied to the entire range of processes that result in the formation of sedimentary rock, from initial erosion through sediment transport and settling to the lithification of the sediments. However, the strict geological definition of sedimentation is the mechanical deposition of sediment particles from an initial suspension in air or water. Sedimentation may pertain to objects of various sizes, ranging from large rocks in flowing water, to suspensions of dust and pollen particles, to cellular suspensions, to solutions of single molecules such as proteins and peptides . Even small molecules supply

200-642: Is the largest lake in Madagascar , located in Alaotra-Mangoro Region and on the island's northern central plateau. Its basin is composed of shallow freshwater lakes and marshes surrounded by areas of dense vegetation. It forms the center of the island's most important rice-growing region. It is a rich habitat for wildlife, including some rare and endangered species, as well as an important fishing ground. Lake Alaotra and its surrounding wetlands cover 7,223 square kilometres (2,789 sq mi), and include

220-472: Is the type locality of the butterfly Artitropa alaotrana and an important but increasingly threatened habitat for waterbirds , including the endangered Meller's duck ( Anas melleri ). Two waterbird species were endemic to northern Madagascar, the Madagascar pochard ( Aythya innotata ) and the Alaotra grebe ( Tachybaptus rufolavatus ). The Madagascar pochard is now critically endangered and no longer found on

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240-446: The attenuation of waves or currents, promoting the settlement of suspended particles. An undesired increased transport and sedimentation of suspended material is called siltation , and it is a major source of pollution in waterways in some parts of the world. High sedimentation rates can be a result of poor land management and a high frequency of flooding events. If not managed properly, it can be detrimental to fragile ecosystems on

260-422: The fluid in which they are entrained and come to rest against a barrier. This is due to their motion through the fluid in response to the forces acting on them: these forces can be due to gravity , centrifugal acceleration , or electromagnetism . Settling is the falling of suspended particles through the liquid, whereas sedimentation is the final result of the settling process. In geology , sedimentation

280-462: The lithification of the sediments. However, the term is more particularly applied to the deposition of sediments, and in the strictest sense, it applies only to the mechanical deposition of sediment particles from an initial suspension in air or water. Sedimentation results in the formation of depositional landforms and the rocks that constitute the sedimentary record . The building up of land surfaces by sedimentation, particularly in river valleys,

300-399: The bottom of the vessel, forming sludge or slurry at the vessel base. Settling is an important operation in many applications, such as mining , wastewater and drinking water treatment, biological science, space propellant reignition, Classification of sedimentation: When particles settling from a suspension reach a hard boundary, the concentration of particles at the boundary

320-405: The catch, perhaps because it was moving into an empty ecological niche as a phytophagous species. The fertile plain surrounding Lake Alaotra is Madagascar's most important rice-producing region. The hills surrounding the lake were formerly forested but have mostly been cleared for farmland in past decades. Severe erosion on these vulnerable hill slopes has caused considerable sedimentation of

340-478: The lake, although very small numbers exist elsewhere. The Alaotra grebe was declared extinct in 2010. The area of the lake may have been its only habitat. The Ambato River provides the lake with water and also drains it. After 381 km, the river flows into the Indian Ocean . Sedimentation Sedimentation is the deposition of sediments . It takes place when particles in suspension settle out of

360-625: The lake, which is fast disappearing; the lake is now only 60 cm (24 in) deep during the dry season. Pressure to create more rice fields has also led locals to burn the reedbeds surrounding the lake. These reedbeds provide the sole habitat of the endemic Alaotra gentle lemur ( Hapalemur griseus alaotrensis ). The Alaotra gentle lemur is now limited to only 220 km (85 sq mi) of remaining reedbeds, and in recent years, its population rapidly declined by 60%, from about 7,500 individuals in 1994 to 3,000 in 2001, mostly from habitat loss, but also from hunting by local villagers. The lake

380-444: The non-native water-hyacinth Eichhornia . 44 species of birds have been observed at the lake, including ten Madagascar endemic species. Native birds include Meller's duck ( Anas melleri ), Madagascar grebe ( Tachybaptus pelzelnii ), Madagascar flufftail ( Sarothrura insularis ), Madagascar pond heron ( Ardeola idae ), Madagascar snipe ( Gallinago macrodactyla ), and Madagascar marsh harrier ( Circus macrosceles ). The lake

400-572: The northwestern corner of the lake. The Lily is a tributary of the Sakay River , which is a tributary of the Tsiribihina River . The Tsiribihina flows westwards to empty into the Mozambique Channel . There are reedbeds along the edges of the lake, composed mostly of species of the reed Phragmites and the sedge Cyperus , which grow up to two meters high. There are also floating mats of

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