Tigris

Long ago, huge pieces of Earth's crust pushed into each other. Tall mountains rose up.

In some places, the Tigris may have kept cutting down as the land rose.

The Tigris begins on the Eastern Anatolian Plateau, a high land that rose when the African and Arabian plates began colliding with the Eurasian plate. The river has been shaping its valley for about 13 million years, since the plateau was lifted.

Near its source, the Tigris cuts across the grain of strongly deformed metamorphic rock in the Eastern Taurus Mountains. Geologists think parts of the river may have kept their old course while the land rose, because the river cut down faster than the land was lifted.

The Eastern Anatolian Plateau belongs to the active Alpine-Zagros-Himalayan mountain belt, which gained its relief as the Southern Neo-Tethyan Ocean closed and the African and Arabian plates collided with Eurasia. The Tigris has been geomorphically active for about 13 million years, since the plateau's uplift.

Nicoll describes three kinds of reach. In the headwaters, the river is transverse, cutting across the structural grain of metamorphic massifs in the Bitlis-Zagros Suture Zone. Where the Arabian plate meets the suture, it may be antecedent: a course set in the Miocene and kept during uplift because incision outpaced uplift. Downstream it crosses a carbonate platform and then Tertiary-Quaternary basalt flows north of Diyarbakır, which buttress its right bank.

In some places, the rock under the river is full of salt and limestone.

Water slowly dissolves it. Holes can open up in the ground.

East of the city of Diyarbakır, the Tigris turns to flow east-west. Here it wears through cracked limestone, shale and gravelly rock mixed with thin layers of salt.

Water dissolves these rocks underground, a process called karst. It widens the valley and makes sinkholes and other collapse features. Near the village of Bismil, geologists have mapped sinkholes and hollows, and new sinkholes formed there in the years before the study.

East of Diyarbakır the river trends east-west, eroding fractured Miocene and Pliocene limestones, shales and conglomerates thinly interbedded with salts. There, bedrock erosion and valley widening are controlled by karst processes in evaporite and carbonate rock (Schumm and others, 1995).

Mapping east of Bismil shows sapping, piping and subsurface collapse, including dolines, sinkholes and potholes; sinkholes and groundwater-sapping features developed during the decade before the vignette was written. Tectonic uplift, deformation and rock type are the main controls on the upper Tigris, with climate adding to erosion.

Winters in the mountains are cold and snowy. Summers are hot.

Melting snow and ice flow into the rivers. People use that water to drink.

The upper Tigris region is semi-arid, with hot summers and cold winters. About 580 millimeters of precipitation falls in a year, mostly as snow. The land is a cold steppe with woody shrubs.

Melting ice and snow from the high plateau feed the local rivers and are the main drinking water source for the many people who live there. At Diyarbakır, UNESCO protects the Hevsel Gardens, a green strip between the old city and the Tigris that supplied the city with food and water for centuries.

Nicoll describes the upper basin's modern climate as semi-arid, with hot summers and cold winters and an average of about 580 mm of precipitation, mostly as snow; the natural vegetation of this cold-steppe landscape includes woody shrubs and occasional Mediterranean elements. Snow and ice melt from the plateau's highlands feeds local drainages and is the main source of drinking water for the large local population.

UNESCO's Diyarbakır Fortress and Hevsel Gardens Cultural Landscape (inscribed 2015), on an escarpment of the Upper Tigris basin, includes the Hevsel Gardens, a green link between the city and the Tigris that supplied the city with food and water, along with the Ten-Eyed Bridge.

The river's water comes from melting snow, and many people drink it.

So visitors keep the river clean and take their trash home.

Because melting snow and ice from the highlands are the main drinking water for many people in the upper Tigris region, keeping the river and its banks clean matters. Taking all trash home is PlayOwl's simple tip.

In karst areas, the ground can collapse into sinkholes, and geologists have seen new ones form in recent years. That is a good reason to stay on paths and roads near the valley, another PlayOwl tip built on the geologists' findings.

Two facts from the SERC vignette shape PlayOwl's advice. First, snowmelt from the plateau is the main source of drinking water for the large local population, so visitors should leave the river and its banks clean. Second, karst processes east of Diyarbakır have produced sinkholes, piping and subsurface collapse, with new sinkholes and groundwater-sapping features developing in the decade before the study, so staying on established paths and roads near the valley is sensible.

These are PlayOwl's recommendations drawn from the scientific description, not official rules.