Flowing (Lotic) Systems
The US has more than 3.5 million miles of flowing water systems, which include springs and seeps, rivers, streams, creeks, brooks and side channels.
At left, the Four-Dimensional Concept (Ward 1989) recognizes that lotic systems' structure exists in a four-dimensional framework, as below:
- Longitudinal (in an upstream and downstream direction) - Flowing water systems commonly go through structural changes en route from their source to mouth. Three zones are usually recognized - headwaters, where flow is usually lowest of anywhere along the system, slope is often steepest, and erosion is greater than sediment deposition; transfer zone, the middle range of the stream where slope usually flattens somewhat, more flow appears, and deposition and erosion are both significant processes; and the downstream end's depositional zone, where flow is highest but slope is minimal and deposition of sediment significantly exceeds erosion most of the time.
Lateral (across the channel, floodplains and hillslopes) - Again, significant variation occurs among stream types, but a common pattern includes the channel, the deepest part of which is called the thalweg; low floodplains that are flooded frequently, and higher floodplains (e.g., the 100-year or 500-year) that are rarely inundated; terraces, which are former floodplains that a downcutting stream no longer floods; and hillslopes or other upland areas extending up-gradient to the watershed boundary.
Vertical (surface waters, ground water and their interactions) - It is always important to recognize that water bodies are not purely surface features; rivers and streams constantly interact with groundwater aquifers and exchange water, chemicals, and even organisms. Over its entire length, a stream often varies between influent reaches where surface water leaks downward into the aquifer, and effluent reaches where the stream receives additional water from the aquifer.
Temporal (through time, from temporary response to evolutionary change) - The dimension of time is important because rivers and streams are perpetually changing. Structure as described in the other three dimensions above should never be considered permanent, and watershed managers should always think of structure not just as what is there now, but in terms of the structural changes in progress and their rates of occurrence.
Recognition of different types of streams and rivers is mostly reliant on channel form and function. For more on stream and river categories see classification of stream types.
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