Achra

Achra is a geological depression in central Charnea named for the Chott Achra salt lake at its base. The Achra depression has a land area of 45,600 square kilometers forming a broad trough in the floor of the Ninva desert extending in a southwest to northeast direction parallel to the Agala mountains to the south. The region is notable for the inland delta formed by the flow of the Iza river as it drains into the Achra basin, creating an area of wetlands, permanent and ephemeral lakes and floodplains in stark contrast to the surrounding desert landscape of the Ninva. Achra is one of the most developed regions in Charnea due to the abundance of arable land of the Iza delta combined with its central location at the crossroads of the Ninva as well as the availability of important natural resources in the surrounding area. Achra has a population of 14 million, accounting for more than half of the Charnean population, and plays host to the urban areas of Greater Agnannet as well as Asihar and Atafala that together form the Achra Triangle megacity.

Geology

The Achra depression is a structural foreland basin formed by the same geologic processes responsible for the uplift of the eastern Agala highlands immediately south of Achra. All of Achra lies below sea level, with an average elevation of 55 meters below sea level. The lowest point in the depression, which is also the lowest point on the Scipian continent, is the bottom of Chott Achra which lies 167 meters below sea level. Geological evidence, primarily in the form of non-marine evaporite fossil salts and rock formations, indicate that Chott Achra was a much larger lake in the prehistoric past, fed by a variety of now-extinct watercourses responsible for the formation of the ravines and canyons found in the badlands landscape of northern Achra and the neighboring regions of the Ninva desert. The general geology of Achra is of a sedimentary nature, consistent with much of the Ninva desert. The sandstone bedrock of Achra provides a highly permeable medium for the Ninvite Sandstone Aquifer, a porous aquifer regenerated primarily by the infiltration of the Iza river's floodwaters as they spread across the floor of the Achra basin. Although the Ninvite Sandstone Aquifer extends across much of the central Ninva desert, its most accessible areas are found in the low-lying Achra depression.

Porous carbonate deposits of biological origin can be found in lower layers of the Achra bedrock, linked to the shallow lacustrine environment of the greater Chott Achra inland sea that existed several million years ago. These strata are the most productive horizons for the petroleum extraction industry in central Charnea. Igneous intrusive rock can be found in Achra, with the two significant igneous formations penetrating the sedimentary layers in the north and in the southeast of the wider Achra region. The ancient volcanic activities responsible for these igneous intrusions are also responsible for the porphyry orebodies of hydrothermal origin that account for a significant portion of the mineral wealth that can be found in Achra.

Iza delta

The Iza river carries rainwater from the Agala highlands into the Ninva desert, draining into the Achra basin from the south into five main channels and multiple floodplains and fluvial wetlands and marshes. The Iza delta varies in size on a seasonal basis, extending across 14,000 square kilometers between July and September when heavy rains in the Agala cause major downstream flooding along the course of the Iza. During the height of the dry season between December and March, the delta can recede to as little as 2,500 square kilometers. The waters of the Iza tend to remain along the deepest channels of the delta, while the shallower channels and wetlands are only flooded in the rainy season. All of the water carried by the Iza into the Achra basin is ultimately lost to evaporation or infiltration into the sandstone aquifer. Salt lakes and salt marsh environments can be found at the extremes of the Iza delta's maximum extent in central Achra where floodwaters spread out across wider depressions and evaporate to leave behind bodies of water with high salinity or fully dried out salt pans over the dry season before being again flooded by the Iza the following August. The largest of these saline bodies is Chott Achra itself, the salinity of which varies from brackish (1-3%) to a fully saline environment (3-4% salinity) in some parts of the lake during the dry season.

The floodplains of the delta region represent a naturally irrigated and fertile area of arable land which has been exploited since antiquity. The traditional method of agriculture in the Iza delta is a type of flood recession cropping in which crops are planted directly in the moistened beds of the flooding channels after floodwaters recede toward the end of September. This method is relatively inefficient and vulnerable to instances of severe flooding that delay the recession of the floodwaters by weeks or months, but is still practiced in the modern day in some parts of the delta. Modern systems of flood control including a number of dykes, artificial channels and reservoirs have been implimented primarily to provide a stable year-round water supply to the urban regions surrounding the delta, but have also been beneficial for more sophisticated and efficient agricultural methods in the delta. The extensive development of the Iza delta as the breadbasket of Charnea has significantly disrupted many elements of the natural ecosystem caused primarily by the diversion of floodwaters for irrigation and water projects, causing significant shrinking of Chott Achra and other northern salt lakes over the decades.