Nearctic Upland Rivers - Code: Ne11F

At the most basic level a river is a flowing body of water that drains the surrounding landscape. These bodies of water may be permanent, semi-permanent or seasonal depending on the hydrology of the region but for the purposes of this book, we will be talking principally about permanent rivers. The terms stream, creek, and river are all used for these bodies of water and though they do not have strict definitions, creek and stream tend to refer to smaller bodies of water. Smaller streams join to form mid-sized streams which in turn form rivers. The network of waterways forming a single terminal drainage point are termed a watershed. Some watersheds can encompass tens of thousands of square miles of land. Creeks and streams that do not have any smaller bodies flowing into them are termed headwaters. Headwaters tend to be relatively free of both pollution and predators and are important for specific wildlife.
Unlike most habitats in this book, the most important factors for river wildlife are all abiotic – waterflow, temperature, light, suspended matter, and bedrock define a river. Ecologically, rivers are typically divided into Upland Rivers and Lowland Rivers based on these abiotic factors. While most Nearctic Upland Rivers are mountainous in origin, they are not exclusively so. The most important factors in the formation of upland rivers are the velocity of water and the gradient of the slope the waterway follows. Upland rivers have swift flowing water and a relatively steep gradient. This tends to produce a similar suite of characteristics that unify upland rivers including higher oxygen content in the water, cooler water temperatures, and a riverbed dominated by rocky surfaces. Upland rivers also tend to be more structurally complex and frequently have an alternating pattern of riffles and pools. The surrounding topography and high velocity mean that upland rivers tend to be more incised. The level of incision refers to how a river has eroded the surrounding bedrock. Heavily incised rivers tend to have steep canyons or gorges associated with them.
The swift flowing nature of upland rivers means few aquatic plants can grow here. Primary nutrient input for this system is usually in the form of algae growing on rocks when exposed to adequate light. In upland river systems covered by a canopy, falling detritus, especially leaf litter, is the primary nutrient input.
Waterfalls occur in areas where rivers flow over a vertical drop or a series of steep drops. These formations are a popular visual spectacle and provide important microenvironments in upland river systems.