Lake Urmia, Iran

Historical and geological  context

Lake Urmia, formerly Lake Rezaiyeh, is a shallow perennial saline lake in the lowest part of an endorheic depression in Azerbaijan Province of northwest Iran. Surface waters have a current elevation around 1,270 to 1,280 m asl and surface salinities range annually from 80 to 340‰.

Historically the lake was some 140 km long and 80 km wide, with an area between 4,700 - 6,000 km2 (Warren, 2016). Urmia Lake was the second largest hypersaline lake in the world and the largest terminal lake in West Asia prior to catastrophically losing about 90% of its surface area over the last few decades as a consequence of over-extraction of its inflow waters for drinking and agricultural use. Average water depth was around 5 metres, which deepened to 8 metres in the south, where the maximum water depth was around 15 metres. 

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Figure 1. Osman’s fist, Lake Urmia, Iran

Lake Urmia lies at the bottom of an endorheic basin located further along the same alpine collision suture as the Turkish lakes. It was created as the Turkish and Eurasian plates were squeezed sideways along very active strike-slip fault systems. Palaeozoic metamorphics crop out in the Zagros Mountains to the west of Lake Urmia. Infracambrian sediments lie to the south, while the northern and northeastern lake margins are characterised by outcrops and subcrops of the evaporitic Middle Miocene Fars Formation and underlying marine limestones of the Lower Miocene (Figure 1). Several Miocene salt domes pierce the surface of the lake’s drainage basin.

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Figure 2 Lake Urmia, Iran contrasting in the extent of lake waters between 1998 and 2014, Also shown are positions of Core site BH2, the 1965 (low) and 1977 (high) shorelines and the extent of Late Pleistocene Lake Urmia. In part after Kelts and Shahrabi, 1986).

Lake sediments

Lake floor sediments are mostly aragonitic pelletal laminite/mudstones, near-identical in appearance, depths of deposition and origin to those in Great Salt Lake, Utah. The Urmia pellets are composed of 80% aragonite, along with small amounts of quartz and calcite, and rarely dolomite (Eimanifar and Mohebbi, 2007). Laminites in both lakes result from seasonal biologically-mediated precipitation of aragonite (Kelts and Shahrabi, 1986). When salinity and nutrient levels are suitable, Urmia’s stratified surface waters support an abundant summer flora of the green alga Enteromorpha intestinalis, along with species of Dunaliella and Tetraselmis. Blooms can be so intense that in some years the water changes colour to a deep green and light penetration is measured in tens of centimetres. The algal bloom is followed quickly by a bloom of the endemic brine shrimp Artemia urmiana (up to 4,000 individuals per litre), which graze the alga. While feeding, the shrimp also ingest particulate aragonite that is precipitating in the lake waters. Thus aragonite in the Urmia laminites occurs both as 10 µm-long euhedral prisms in pelagic muds and as the principal constituent in sand-sized brine shrimp faecal pellets (Figure 3). Both types of aragonite are interlaminated with organic-rich laminae in a matrix dominated by terrigenous sediment brought to the lake by the spring melt. The aragonite is sometimes sufficiently cemented to form cm-thick crusts on and beneath the lake floor. The precipitation of aragonite, rather than calcite, in the lake reflects the high Mg/Ca ratio of the waters (molar ratio 28:1).


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Figure 3. Sediments in the lake are dominated by aragonite especially faecal pellts from brine shrimp (after Sharifi et al., 2018)

At the northern end of the lake, the gypsum bed is now some 1.5 metres below exposed laminites, with the latter tied to the same sedimentation style as the current subaqueous lake floor. The gypsum bed was interpreted as a sabkha unit by Kelts and Shahrabi, (1986). That is, earlier in the Quaternary, the lake level was lower than today and portions of the lake floor became so saline that gypsum precipitated in a saline playa. Approximate U-Th dates based on aragonite-pellet lamina across a 100-m long piston core, studied by Stevens et al. (2012), places the playa episode at 15-55 ka. In other parts of the lake, partially exposed dolomitic mudflats were probably time equivalents to this now buried gypsum. Oolites shoals formed at the northeast end of the lake but they too are now buried beneath 30 cm of putrid, black organic-rich mud.

The biota and the broad spectrum of carbonate pelletal sediments found in Lake Urmia are near identical to that of Great Salt Lake, Utah. Like the Great Salt Lake, sediments accumulating in the deeper parts of the perennial saline lake are pelletal aragonite laminates. In shallower waters, the sediments can construct aragonitic ooid and pelletal strandzones (as near Gulman Khaneh on the western margin) or cemented crusts/beachrock dominated by aragonite pellets, as typifies much of the lake edge.

Climatic setting and water levels

Climatically, the lake lies in a semi-arid, desert steppe environment (Köppen BSk), with precipitation averaging 30 cm per year. The climate is harsh with winter temperatures down to -20°C and summers up to 40°C. Surface inflow from the 52,000 km2 drainage basin is strongly seasonal, driving an annual lake water level fluctuation of up to 1-2 metres, especially during the spring melt when the Talkheh and Simineh Rivers discharge around 57 m3/s into the lake. By summer the inflow drops to 1.7-3.7 m3/sec, and lake levels fall once more.

Topographically, the surrounding region is either plateau or mountain. Mountains west of the lake have peaks that exceed 3350 metres making that side of the lakeshore somewhat steeper. Salt flats and the fan deltas characterise lake sediments along the southern and western shores. Slight changes in water level can drive drastic migrations of the strandline across the evaporitic mudflats of the lake. In 1970 the lake had a water level of 1277 m asl, while in 1965 the water level had hovered around 1274 m; the lake level had risen by 3 metres in a decade, and the shoreline had migrated some 5-10 km (Figure 2). From 1996 until 2017, there was a 7-metre fall in lake level that plateaued with the end the drought in 2018 (Figure 4). 

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Figure 4. Lake Urmia changes in water level and TDS g/L (replotted from Sima et al., 2020)

This meant by 2017 the lake’s water level had reached historic lows, while salinities have doubled since the mid 1990s and were attaining highs of 300-340 g/l approaching halite saturation (Figure 4). This scenario of ongoing falling water levels and elevated salinities is likely due to over-extraction from inflow water sources and inefficient irrigation methods. Agricultural lands and urban areas in Lake Urmia basin have increased by 98% and 180% from 1987 through 2016 (Sheibani et al., 2020). High levels of water consumption mainly for irrigational purposes, coupled with deficient methods of irrigation, resulted in an increase in evapotranspiration and over-exploitation of incoming rivers which led to a 56% decrease in the amount of water that flows via rivers to the lake from 1966-1996 to 1997-2012. Some 13 rivers that supply the lake are now dammed and feed stored waters into various irrigation schemes.  This combination of defines an anthropomorphic hydrological/salinisation evolution that is detrimental to the diversity of the lake biota, much like the anthropogenic pressures that detrimentally affect biodiversity in other modern saline sumps such as the Dead Sea, the Aral Sea and the Coorong Lakes.

The stress of a rapidly growing population, currently around 6.5 million people within the Urmia watershed, will likely force continued diversions and damming of streamflow within the basin before it reaches Lake Urmia sump. However, the Iranian government has instigated water efficiencies in the surrounding agricultural regions. This improvement in agricultural practices, and the 2018 floods that ended the drought years, means that as of mid-2020 the water covered area of the lake has risen to more than 3,200 square kilometres, showing an increase of more than 800 square kilometres compared to the situation a year earlier. It remains to be seen if the governmental changes to the over-exploitation of the inflow drainages will be effective in maintaining improved lake levels in future drier times (Figure 5, 6).

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Figure 5. Idle shipping due to falling Urmia water level (taken 2014)
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Figure 6. Saline dust deflating from the desiccated mudflats of Lake Urmia (Effati et al., 2018)

References

Effati, M., et al. (2018). "Application of Satellite Remote Sensing for Quantification of Saline Dust Emission Probability in the Lake Urmia Watershed in Iran." Conference: University of Arizona, College of Agriculture and Life Sciences Poster Forum, Tucson, AZ.

Eimanifar, A. and F. Mohebbi (2007). "Urmia Lake (Northwest Iran): a brief review." Saline Systems 3(1): 1-8.

Kelts, K. and M. Shahrabi (1986). "Holocene sedimentology of hypersaline Lake Urmia, northwestern Iran." Palaeogeography, Palaeoclimatology, Palaeoecology 54(1-4): 105-130.

Sharifi, A., et al. (2018). The Vanishing of Urmia Lake: A Geolimnological Perspective on the Hydrological Imbalance of the World’s Second Largest Hypersaline Lake. The Handbook of Environmental Chemistry, Springer Nature Switzerland: 1-38.

Sheibani, S., et al. (2020). "Influence of a lake-bed sediment deposit on the interaction of hypersaline lake and groundwater: A simplified case of lake Urmia, Iran." Journal of Hydrology 588: 125110.

Sima, S., et al. (2020). " "Managing Lake Urmia, Iran for Diverse Restoration Objectives: Moving Beyond a Uniform Target Lake Level" (2020). ." Civil and Environmental Engineering Faculty Publications. Paper 3757. https://digitalcommons.usu.edu/cee_facpub/3757.

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