The concept of stream order assigns numerical designations that indicate where in a watershed drainage system a certain stream segment lies. The smallest flows from upland areas, as well as springs and seep sources that maintain defined stream beds throughout the year are first-order streams. Where two first-order streams combine, a second-order stream is designated; and two second-order streams join creating a third-order stream. The hypothetical stream system at left has been labeled to indicate the stream order of the various segments. As a 'real world' example, consider Bobs Creek which, upstream of the State Route 869 bridge near Pavia, is a 3rd order stream and downstream is a 4th order stream because Wallacks Branch, a third-order stream, joins the flow immediately downstream of the bridge. At Reynoldsdale, Bobs Creek joins Dunning Creek, itself a 4th order stream at this point, to create a 5th order stream from there downstream. Typically, 3rd and 4th order streams comprise the majority of our quality trout streams.
The Raystown Branch is a sixth-order stream which joins with others to form the 7th order Juniata River. The Susquehanna River, the longest river along the East Coast, is an 8th order stream. There is but one 10th order river in the United States, the Mississippi.
The River Continuum Concept
Streams are continuous gradients where downstream processes are linked to upstream events, producing a predictable change in the assemblage of organisms along the stream sysytem. The River Continuum Concept, developed by Dr. Robin Vannote and others at Stroud Water Research Center in the 1980s, is a model for describing and understanding characteristics of a watercourse as it flows from the headwaters to the mouth. Streams are open dynamics systems, interacting with the land, through changing influences of soil chemistry, vegetative cover, and topography. Changes in stream slope effect stream depth and width, water velocity, and streambed composition. Soil chemistry has significant effects on the chemical composition of small headwater streams due to the greater proportion of the flow being in contact with the stream banks and bottom in these smaller waters. The amount and type of vegetation along the stream, as well as throughout the riparian zone, influences water temperature, soil stability, and community of aquatic plants and animals. The River Continuum Concept is a useful tool in environmental assessments of stream corridors but has limitations when applying to certain stream types, especially those away from the eastern woodland streams where it was developed. For example , many western streams begin as headwaters in rather barren, mountainous terrain devoid of the leafy and woody energy input typical of our eastern waters. Such headwaters are naturally low in mineral and nutrient content and will have low diversity and density of aquatic organisms.
Limestone spring creeks are another special case. Those stream sections that arise from valley floors, as the smallest, unbranched tributaries of the system, would typically be labeled as 1st order streams. In some places these "1st order" creeks arise from a spring as rather large flows, more typical of maybe a 3rd order stream. The characteristics we expect to find in low-order headwater streams, described below, are not features usually found in low-order spring streams in limestone valleys.
In Bedford County, many watersheds have sections characteristic of limestone spring creeks, with other stream segments, or tributaries, of the 'freestone' type. "Freestone" is a term used frequently in the trout-fishing, especially fly-fishing, lexicon to refer to a stream that is fed by run-off and spring seeps from uplands without the influence of springs fed by limestone aquifers. Freestone streams are more common than limestone streams but in some areas of Pennsylvania, limestone is prevalent, particularly in valleys.
Headwaters are streams of first, second, and third order and in natural condition are usually narrow with thick vegetation along the banks. Heavy shading reduces the photosynthetic potential for aquatic plants and most organic material enters the stream as fallen leaves, twigs, or branches. Aquatic macroinvertebrates that process this organic material by breaking it into smaller bits for consumption are known as shredders and include many mayfly and stonefly species. Collector type organisms that utilize coarse particulate organic material may also be well-represented. These streams are often home to the coldwater fish species such as sculpins, black-nose dace, longnose dace, and brook trout. (Click here for a chart of stream organism feeding group descriptions.)
First and second order streams are typically higher gradient streams. First order streams from headlands usually flow rather straight, directly down the slope with little meandering, as a series small pools and cascades. As slope decreases, and the stream widens with additional tributary flows, third order streams will have more of a riffle-pool character with some meandering.
The middle reaches are comprised of 4th through 6th order streams. The streams are wider in these sections so even with natural undisturbed riparian vegetation, more sunlight gets to the water and the larger rocks and logs can support films of algae and bacteria, known as periphyton. Production through photosynthesis exceeds consumption by respiration. A smaller portion of the organic material is composed of leaves and such and more fine organic particles are part of the drift. Collector and grazer organisms make up a significant part of the aquatic invertebrate community, with an increase in number and diversity of caddisflies and additional populations of fly larvae and certain mayfly species.
Middle reach streams can host coldwater fish species such as trout if well-shaded or fed by significant spring sources but are likely to also be home to coolwater species such as smallmouth bass, fallfish, and suckers, or even warmwater species such as largemouth bass and sunfish. Middle reach streams are much more likely than headwater streams to be impacted by both point and non-point sources of pollution. Residential, urban, and agricultural landscapes are likely to be features of these watershed areas with run-off impacting aquatic community health and, possibly, patterns of flood and high water damage. These middle reach streams are more likely to be recreational areas for a broader variety of uses, including fishing, small watercraft boating, and swimming. The downstream, main stem portions of most of our 'named creek' watersheds are in this category.
Lower reaches are of 7th order and greater. These large rivers drain areas of thousands to more than a million square miles (Mississippi River). Fine particulate organic material (FPOM), in the drift from upstream segments, remains a feature and an important energy input. Plankton organisms are prevalent and contribute to increased turbidity that limits photosynthesis by rooted plants in deeper water sections.
The large rivers of the lower reaches are important transportation resources and often support commercial fisheries as well as sport fishing and recreational boating.
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