Major Natural Factors Research Articles

Assessing the Effect of Natural Controls and Land Use Change on Sediment Yield in a Major Andean River: The Magdalena Drainage Basin, Colombia

The Magdalena River, a major fluvial system draining most of the Colombian Andes, is a world-class river, in the top 10 in terms of sediment load (approximately 150 MT/yr). In this study, we explore the major natural factors and anthropogenic influences behind the patterns in sediment yield on the Magdalena basin and reconstruct the spatial and temporal pattern of deforestation and agricultural intensification across the basin to test the relationships between land use change and trends in sediment yield. Our results show that sediment yield for the whole Magdalena catchment can be explained by natural variables, including runoff and maximum water discharge. These two estimators explain 58% of variance in sediment yield. Temporal analyses of sediment discharges and land use show that the extent of erosion within the catchment has increased over the last 10 to 20 years. Many anthropogenic influences, including a forest decrease by 40% in a 20-year period, an agriculture and pasture increase by 65%, poor soil conservation and mining practices, and increasing rates of urbanization, may have accounted for the overall increasing trends in sediment yield on a regional scale.

Hawai'i Coral Reef Assessment and Monitoring Program: Spatial Patterns and Temporal Dynamics in Reef Coral Communities

The Hawai'i Coral Reef Assessment and Monitoring Program (CRAMP) was established to describe the spatial and temporal variation in Ha- waiian coral reef communities in relation to natural and anthropogenic factors. Sixty permanent reef sites stratified by depth have been monitored in the main Hawaiian Islands since 1999 and formed the basis for analysis of temporal change over the initial 3-yr period. A rapid assessment technique (RAT) was developed to supplement the monitoring site data and provide much wider geographic coverage, but with a focus on spatial patterns rather than temporal change. Analysis of these data supports and amplifies the results of many other ecological studies on Hawaiian reefs. The data revealed that the major natural factors influencing reef coral community structure in Hawai'i include depth, wave height, wave direction, island age, rugosity, and sediment grain size. Pos- sible anthropogenic influences and trends also appeared in the data. Areas of decline appear to be concentrated on islands with high human population or in areas suffering from extensive sedimentation. Reefs receiving high terrigenous runoff contain sediments with high organic content. Spatial analysis showed an inverse relationship between percentage organics and coral species richness and diversity. Reef coral communities can undergo natural oscillations over a period of years, so continuation of the CRAMP longer-term monitoring is required to establish long-term (decadal) environmental trends.

Impact of environmental change on the freshwater fish Fauna of Iraq

Natural and man‐made factors have shown the ability to change the freshwater fish environment in Iraq over the last 6000 years. Changes in climate were among the major natural factors that caused a significant alteration in the distribution and abundance of freshwater fish in Mesopotamia. However, man‐made factors contribute more to the impact on the freshwater system in Iraq and ultimately on the freshwater dwellings, e.g. fish. There are a number of industrial factors, such as water abstraction by dams, devegetation, heavy fishing, pollution and species introduction, that have played a major role in the destruction of the freshwater fish habitat in Iraq and as a result real and more effective conservation measures need to be proposed.

Recent and future changes in Arctic sea ice simulated by the HadCM3 AOGCM

The HadCM3 AOGCM has been used to undertake an ensemble of four integrations from 1860 to 1999 with forcings due to all major anthropogenic and natural climate factors. The simulated decreasing trend in average Arctic sea ice extent for 1970–1999 (−2.5% per decade) is very similar to observations. HadCM3 indicates that internal variability and natural forcings (solar and volcanic) of the climate system are very unlikely by themselves to have caused a trend of this size. The simulated decreasing trend in Arctic sea ice volume (−3.4% per decade for 1961–1998) is less than some recent observationally based estimates. Extending the integrations into the 21st century, Arctic sea ice area and volume continue to decline. Area decreases linearly as global‐average temperature rises (by 13% per K), and volume diminishes more rapidly than area. By the end of the century, in some scenarios, the Arctic is ice‐free in late summer.

Sediment transport in the Cauvery River basin: sediment characteristics and controlling factors

During the monsoon period, 75% of the annual rainfall, 73% of the annual water discharge and 85% of the annual sediment transport takes place in the Cauvery River. Basin geology and river water discharge are the major natural factors controlling the sediment transport. Sediment transport in the Cauvery River is greatly influenced by the presence of two major dams built across the river. An examination of the particle-size distribution of the suspended sediments shows that the less than 20 μm fraction accounts for nearly 60% of the total sediment transported by the river. At dam sites, coarse sediments are selectively removed from the suspended load. The suspended mineralogy is dominated by quartz and feldspar, followed by clay minerals and carbonates. Clay minerals average about 30% and are dominated by the montmorillonite type. The results of our study also demonstrate the importance of such sediment characteristics as particle size and mineralogy in controlling the trace metal distribution in sediments.