Drove up to Lake Sevan yesterday. (Claude and the boys are in Germany, so I'm at loose ends.)
Lake Sevan is one of the least known great lakes of the world; if you're not Armenian, you probably haven't heard of it. But it's quite something. It's big -- 70 miles long, 10 miles wide -- and it's high: the lakeshore is at 1900 meters, or about 6400 feet. And it's in a really spectacular setting. Maxim Gorky called it "A piece of sky fallen among mountains", an that's about right: it sits in a huge basin, surrounded by bare stony peaks.
Along the north shore the mountains drop almost straight into the water; along the south shore, there's a broad plain where, um, people have built a lot of ugly hotels. The basin is mostly closed; a lot of little streams flow into the lake, but only one river -- the Hrazdan -- flows out.
Anyway. Up until the 1930s, Lake Sevan was notable for being big, deep, cold, and full of intensely pure water. There was a unique native species of trout. Around the lake was a wide fringe of wetlands, rich in wildlife and an important way-station to millions of migrating birds. There were a few fishing villages, and one small market town, but the huge lake basin was still one of the world's wild clean places. Then the Soviet authorities took an interest in the lake.
You can guess what's coming next.
First the Soviets built a hydroelectric dam on the Hrazdan. Okay, no big deal -- the dam provided electricity to light much of Armenia.
Then they started tapping the lake for irrigation water. The level of the lake started to drop, but still no big deal.
Then in the 1950s, they really got going. Soviet engineers decided that they would drain almost all of the lake, removing 90% of the water. Why? Because they were going to turn the lake basin into a vast agricultural plain, irrigated by the water that used to go into the lake. The reasoning (I think) was that Armenia was becoming an industrial region, so no-longer-a-Lake Sevan would provide the rich harvests to feed the workers. The Soviets began digging irrigation canals, pumping water out by the cubic kilometer. Almost as an afterthought, they drained the wetlands.
Before the water level of Sevan started falling, the lake was an oligotrophic reservoir with a slow release of its waters, with a complete renewal every 44.3 years, high water clarity (average of 13-14 meters) and high oxygen levels during the year. As a result of this brutal use of water, the level of the lake has fallen by 19.6 meters, its volume of water from 58.5 billion cubic meters to 33.0 billion cubic meters, and its area from 1416.2 km to 1238.1 km. During the years of most intensive water use (1949-1962), the water level fell 13 meters (1m per year).
From an environmental point of view, this quick reduction in water level played a key role in the destabilization of the lake’s ecological indicators, which led to the following negative consequences: reduction in water temperature stratification, reducing the hypolimnion volume up to 50% in the Small Sevan, and reducing it entirely in the Big Sevan...
"Oligotrophic" means the lake was divided into layers. There was a (relatively) warm upper layer, which varied between freezing in winter and up to 15 degrees Celsius in summer. Then there was a deep, cool layer -- the "hypolimnion" -- which was constantly around 3 or 4 Celsius, not much above freezing. The upper layer got most of the light, while the deep layer held most of the nutrients. The Soviet water projects drained most of the deep layer off and stirred up what was left.
The reduction in the hypolimnion raised the average temperature of the lake by up to two degrees, increasing the longevity and intensity of horizontal and vertical flows. As a consequence, concentrations of suspended and dissolved organic substances increased several times in the upper and middle water layers of the lake; during their oxidation, the concentration of dissolved oxygen in the lake fell from 8.0 to 3.0 mg O2 per cubic meter.
Concentrations of minerals and general nitrogen in the lake increased up to 30 times (from 0.01 till 0.32 g/m3), and the concentration of phosphorus decreased 20 times (from 0.32 to 0.017 g/m3). This led to the intense assimilation of oxygen and phosphorus by macroalgae, which promoted to the height increase. Water clarity, which plays crucial role in physical-chemical and biological processes, decreased by four times (from 13 to 3 m).
Major changes in biodiversity occurred in the biota of the lake... spawning Sevan trout disappeared… In the period from 1993 till 1995, a mass death of whitefish was observed, due mainly to the reduction of their food base… In the shore, as consequence of the drying of more than 1000 hectares of wetland areas, out of the 167 species of endemic and migrating birds once present 18 are now found. The number of mammal species has fallen sharply. At present, there is an intensive process of desertification.
That report is a few years old. How is Lake Sevan doing today? Well, more in a bit...