|MadSci Network: Environment|
Good question. Below I offer some reasons that may provide your class with possibilities to investigate. Erosion can cause lakes to fill up with soils and other solids. The effect of this can be devestating to lakes. In extreme cases a small lake could fill up with this solid material and displace water out but more likely the solids place a huge biological burden on the lake's "ability" to process waste. This type of pollution is often caused by the removal of plants and trees whose roots hold the soil in place. I found a good site for you and your class to do some work on this including some activities at
http://www.deq.state.la.us/assistance/educate/liquid5.htm A contributing cause in the demise of lakes is simply high levels of evaporation and low levels of rainfall, for example during a drought. In the same way that water in a bowl will evaporate if placed outside on a hot day, a lake can slowly dry up. Four years of crippling drought helped cause the drying up of Lake Hamoun in Afghanistan. However there are usually other circumstances, as lakes generally hold vast quantities of water that is not easily evaporated. According to The Toronto Star (22 June as reported in the journal Environment Oct 2002), the other contributing factor in this case was the Taliban Administration's decision to dam the Hirmand River. Starving rivers of their much needed water is not unique to Afghanistan. The Walker Lake in Nevada is also going through tough times due to a dam up stream. While the result is devastating to the lake ecosystem, the result is diverted water for thousands of acres of farmland (Dry Times at Walker Lake - A vital wildlife oasis in the sagebrush desert of western Nevada is fast disappearing - water being diverted for agriculture, National Wildlife, Oct-Nov, 2002, by Mark Cheater). In California, Owen's Lake has been dry for some time and is reported to be the USA's largest source of dust. There is now a $250 US million project to reclaim the lake by piping water into it. Why did it become dry in the first place? California pipes mountain-stream water 250 miles south of the lake in the Sierra Nevada, water that would have flowed to feed the lake (Dried-up California lake gets muddy facial, efforts to saturate Owens Lake to reduce dust levels- Science News, Dec 1, 2001, by J. Raloff). There are typically several reasons behind any environmental disaster, although you can usually bank on atleast 1 of them being a man-made or based on some financial decision. But what is the cost of fixing it and even if you are prepared to spend the money, can it ever be the same again? Good luck with your research. I find www.askjeeves.com helpful as well as www.findarticles.com
Try the links in the MadSci Library for more information on Environment .