Upland and lowland

Geographers and ecologists use the terms upland and lowland to describe land. These descriptions depend on elevation, which is the height of the land above sea level.

Lowlands are generally areas with low elevation. Most lowlands are usually no higher than 200 meters. However, some unusual lowlands sit below sea level. The Caspian Depression is a famous example of this. Uplands are much higher, often ranging from 200 to 1,000 meters. These areas can form mountain ranges or steep valleys. Some uplands, like the Mongolian Plateau, can still be relatively flat.

In the study of freshwater ecology, these terms describe river habitats. An upland river drains elevated or mountainous country. These rivers flow onto broad alluvial plains where they eventually become lowland rivers. Elevation is a major factor, but it is not the only one. Scientists look at stream gradient and stream power to classify them. Stream gradient is how much the river's course drops in elevation. Stream power is the actual force of the moving water.

A river with a steep gradient will have high stream power. This force causes the water to flow very quickly. The fast water creates an incised course, which is a deep channel. The river bed in these areas is dominated by bedrock and coarse sediments. You might also see a riffle and pool structure in the water. Because the water moves so fast and comes from high places, it remains cool and clear.

Lowland rivers operate under very different physical conditions. These rivers have a very slow gradient, meaning the land drops very little. Because the slope is gentle, the water has much lower stream power. Instead of a straight or deep path, the river follows a meandering course. This means the water winds back and forth across the land. These rivers are typically warmer than upland rivers. The water is often turbid, meaning it looks cloudy. This cloudiness comes from suspended sediment and organic matter.

These different environments create specialized homes for living things. Upland fish must be strong swimmers to handle the fast current. They often have high oxygen needs and limited temperature tolerances. They also use specialized reproductive strategies so their eggs are not swept away. Invertebrates in uplands often live in the interstices, or the small gaps between coarse sediments. These creatures also require high oxygen levels and cooler temperatures to survive.

Lowland habitats support a different set of species. Lowland fish usually have broad temperature tolerances. They can also survive in water with much lower oxygen levels. Many of these species have breeding strategies adapted to slow-moving, warmer water. Invertebrates in these areas often live in fine sediments. They might also use submerged woody debris, known as snags, or submerged macrophytes, which are water weeds.

Lowland alluvial plains are large areas formed by river processes. These plains form through the deposition of sediment over a very long period. Rivers carry sediment from highland regions and drop it in lowland regions. The American Bottom is a major example of this. It is a large floodplain of the Mississippi River in Southern Illinois. Other examples include the Bois Brule Bottom. In the United States, these areas often host Bottomland hardwood forests. These are deciduous forests found in broad, lowland floodplains.