Freshwater makes up most inland waters. Unlike the ocean, freshwater is not salty and can support a whole different kind of ecosystem. Freshwater can be groundwater (below the surface) and surface water (above the surface). Fresh surface water can be divided into two main categories: standing (lenticStanding water is lentic.) and running (loticRunning water is lotic.) waters. Standing surface water includes lakes, ponds, and wetlands, and running surface waters include streams and rivers.
Groundwater: When precipitation percolates the ground surface and is not absorbed by plants, it becomes groundwater. It fills the cracks, crevices, and pores between rock and soil particles. Groundwater can eventually make its way back to the Earth’s surface by seeping into a spring, stream, lake, or ocean. Rocks that hold a lot of groundwater are known as aquifersAn underground layer of water-bearing rocks.. Aquifers can be composed of sand, gravel, sandstone, or limestone.
Lakes: Lakes are large bodies of water fed by springs, rivers, streams, surface runoff, groundwater, and precipitation, and are entirely surrounded by land. They can form where there is an interruption in water flow such as a barrier like a dam, however, many are formed by past glacial activity and surface water runoff. Lakes can form distinct layers with different characteristics (such as temperature or dissolved oxygen) that provide a habitat for organisms that differ from stream or river habitats.
Wetlands: Wetlands are an area of standing water (usually shallow) that contains cattails or other water-loving plants. They are often transitional lands between terrestrial and aquatic systems, and occur where the water table is at or near the surface. In order to be considered a wetland, an area must have one or both of the following:
- It must support hydrophytes (plants that have adapted to living in soils that are water saturated) at one period throughout the year.
- Land must be covered or submerged with water for at least one period of the year.
Rivers and Streams: Rivers and streams are bodies of fresh, flowing water that typically have higher levels of dissolved oxygen than lakes because their tumbling movement helps to bring air into the water. Rivers and streams support organisms capable of surviving in swift flowing waters. High and low flow in a river or stream can serve many purposes for its ecosystem, as described in the video below. Water in a river or stream flows from the upstreamThe direction against the flow of water in a river or stream. direction to the downstreamThe direction that the water flows in a river or stream. direction.
TributariesA tributary is a stream or river that flows into a larger stream or main stem of a river or a lake. A tributary does not flow directly into a sea or ocean.: A tributary is a stream or river that flows into a larger stream or main stem of a river or a lake. A tributary does not flow directly into a sea or ocean.
The beginning of a river or stream is known as its headwatersHeadwaters are the beginning, or source, of a river or stream.. Headwaters are often marshy areas fed by groundwater or snow/glacier melt. Many smaller streams with different headwaters will join together to form larger streams, and ultimately, rivers.