Siltation

About: Siltation is a research topic. Over the lifetime, 1420 publications have been published within this topic receiving 20983 citations.

Fish Communities of Midwestern Rivers: A History of Degradation

Abstract: Since humans first crossed the Bering Strait land bridge, growing human populations and their increased technological impact have degraded water resources in North America. Initially, this degradation was transitory and local, but as population centers developed and urbanization increased, many areas developed chronic water resource problems. In the Laurentian Great Lakes, for example, sport and commercial fish resources declined following selective overfishing; extensive watershed modifications, including drainage of lake-margin wetlands; introduction of exotics, especially the sea lamprey, alewife, and salmonids; and progressive chemical modification of lake environments (Emery 1981, Francis et al. 1979, Smith 1972, Wagner and Stauffer 1982). Chemical change began with an acceleration of natural processes (e.g., nutrient enrichment and siltation of shallow areas) and was compounded by drainage of wetlands, discharges of synthetic chemicals and chemical wastes, and increased water temperatures stemming from other technological changes. Each lake's fishery was more or less altered, depending on the size, geographic location, and relative influence of these factors on each lake. Watershed modification and chemical contamination, in particular, have severely altered tributary streams

Reservoirs in Tigray (Northern Ethiopia): characteristics and sediment deposition problems

Abstract: In Tigray (Northern Ethiopia, significant achievements were made, mainly from 1994 to 2002), on the development of agriculture through irrigation by employing seasonally harvested runoff using earth dams. However, most of the implemented schemes are not serving the intended purpose well because of constraints associated with both pre- and post-implementation. Sediment deposition in reservoirs is a serious off-site consequence of soil erosion in the region. However, the extent of the problem is not well studied. Moreover, there are no sufficient and reliable sediment-yield data for Northern Ethiopia, which are important for designing new reservoirs and for implementing soil conservation practices. This study addresses those problems by undertaking: (1) a survey and evaluation of the general characteristics and problems of 54 recently built reservoirs and the characteristics of their respective catchments; and (2) a detailed sediment survey in ten reservoirs and related catchment characteristics. A field survey in 2002–2003, a review of technical reports and interviews were the bases for this study. Most of the reservoirs are under risk of insufficient inflow, excessive seepage and sediment deposition. These problems are mainly attributed to the use of a poor database on hydrology and sediment yield, and the lack of adaptable methodologies for assessing controlling factors at the planning stage. The reservoir survey, which is the first of its kind in Ethiopia, also indicates that specific sediment yield (SSY) varies significantly between catchments: i.e. from 237 to 1817 t km−2 y−1 with an average of 909 ( ± 500) t km−2 y−1. The high spatial variability is mainly associated with differences in lithology, cover, extent of gully network and human activities. Therefore, adapting an average SSY value for the whole region is not recommended for future planning purposes. The sediment deposition problem is significant: i.e. 70 per cent of the study reservoirs have important siltation problems so that they will end their useful life well before the dam design period. Hence, sediment management in reservoirs could be an effective approach towards maintaining the existing storage capacity. For the realization of sustainable land and water development in Tigray, sufficient and reliable database building, development and standardization of appropriate methodologies for predicting sediment yield and capacity building of designers needs to be given top priority. Moreover, awareness creation among policy makers, donors and beneficiaries is also important for action. Copyright © 2005 John Wiley & Sons, Ltd.

Changes in community structure and biomass of seagrass communities along gradients of siltation in SE Asia

Abstract: The patterns of change in species richness and biomass of Southeast Asian seagrass communities along siltation gradients were compared at different sites in The Philippines and Thailand. Seagrass species richness and community leaf biomass declined sharply when the silt and clay content of the sediment exceeded 15%. Syringodium isoetifolium and Cymodocea rotundata were present only in multispecific meadows, while Enhalus acoroides was the only species remaining in heavily silted sediments. The following ranking of species sensitivity to siltation is proposed (from the least to most sensitive): S. isoetifolium → C. rotundata → Thalassia hemprichii → Cymodocea serrulata → Halodule uninervis → Halophila ovalis → Enhalus acoroides . Positive correlations were found between species richness and both community leaf biomass and the leaf biomass of individual seagrass species. The increase in community biomass with increasing species richness was associated with a more even distribution of the leaf biomass among seagrass species. The relationships between percent silt and clay in the sediment and seagrass community leaf biomass and species richness provide useful dose–response relationships which can be used to set allowable or threshold siltation loads in SE Asian coastal waters, and indicate that species loss from seagrass meadows is an early warning of detrimental siltation loads.