LOTIC ECOSYSTEMS
Rivers and streams rarely display the vertical stratification patterns found in standing bodies of water because of their turbulent flow. Although slight differences in temperature can exist between the surface and bottom waters of deep lotic systems, the greatest changes take place as water moves downstream. Flowing water systems frequently possess greater habitat heterogeneity than lentic systems. They also are more permanent ecosystems on a geological or evolutionary scale. Both heterogeneity and permanence tend to increase species diversity. The area drained by a stream and all of its tributaries is called its watershed. Any rain that falls within the watershed will pass through the main stream channel. The streams occupying a watershed form a hierarchical network of channels that hold increasingly larger volumes of water as you move toward the mouth. Ridges and hilltops act as divides that separate the watershed into individual drainages. Watersheds are therefore composed of many smaller drainage basins. Rowlett Creek, for example, is found in the watershed of the East Fork of the Trinity River. Its headwaters begin four miles west of McKinney in west central Collin County at an elevation of 750ft. The stream and its tributaries flow southeast for twenty-six miles. It is joined by Cottonwood Creek near 14th street between Plano and Murphy. Further south, near Garland, it is joined by Spring Creek. Until the late 1960s Rowlett Creek flowed into the East Fork of the Trinity River in southwestern Rockwall County. In 1970 it was diverted to empty into Lake Ray Hubbard. The perennial stream is intermittent in its upper reaches. The watershed area of 137.6 square miles includes the cities of McKinney, Plano, and Allen in Collin County; Richardson and Garland in Dallas County; and Rowlett in Rockwall County. The moderately steep to gently rolling terrain is surfaced by black land clay over limestone of the Austin Chalk formation.
Because the southern portion of the creek was subject to seasonal flooding, several dams were built on the creek in the late 1960s and early 1970s. During this same period the dramatic growth of Plano, Allen, Richardson, and Garland changed the profile of the Rowlett Creek watershed from primarily rural to highly urban. Figure 1., reflects the increased runoff associated with this urbanization. Figure 1. Mean Annual discharge (ft3/sec) of Rowlett Stream Classification Creek from 1970-1998. When conducting a stream study, it is useful to describe the stream so that readers can get a mental picture of what you’re describing. Stream classification helps to identify similarities and differences among streams. Stream order is a classification of streams based on tributary junctions and has proven to be a useful indicator of stream size, discharge, and drainage area. A stream's order is its rank, or relative position, within the watershed network. On a topographic map showing all intermittent and perennial streams in a basin, the smallest unbranched tributaries are designated order 1. Where two first-order streams join, a second-order stream segment is formed; where two second-order segments join, a third-order segment is formed, and so on. See figure 2., for an example. As stream order increases, other characteristics change, such as channel shape, drainage area, habitat, and biological communities. One difficulty with this classification scheme is in deciding what constitutes a first-order stream, since tributaries may be too small to be seen. Another problem is that it is designed for a dendritic drainage system. In linear, elongated systems, a stream may remain low order while growing atypically large. For example, Cottonwood Creek parallels Rowlett Creek, but has few major tributaries and never becomes higher than 2nd order. Rowlett Creek by comparison has a number of major tributaries and at its confluence with th Cottonwood Creek is a 4 order stream. Figure 2. Classification of a drainage system using stream order.
An alternative method characterizes streams by magnitude. As with stream order, two 1st magnitude streams join to form a 2nd magnitude stream. However, magnitude increases by one for each 1st magnitude stream entering. At each confluence the resulting magnitude will always be the sum of the magnitudes of the conjoining tributaries. Magnitude has the same problem as order in defining a first magnitude stream. It is better at classifying elongated systems and more accurately describes small streams. Magnitude does become cumbersome for larger streams. A 10th
order river like the Mississippi River could have a magnitude of Figure 2. Classification of a drainage system using stream magnitude. over 200. Riffles and Pools Stream channels tend to meander through the watershed forming riffles, pools, and bars in response to changes in geologic conditions that affect water depth and velocity. In places where the streambed rises, a depression forms and water backs up forming a pool. As water velocity decreases within the pool much of the particulate matter settles out. Pools therefore act as areas of deposition. In pools, sediments tend to be finer and more Figure 3. Riffles and pools formation in response to changes in streambed steepness. homogeneous. As water flows out of these depressions, velocity increases as the streambed drops. This portion of the channel where relatively shallow, rapidly flowing water occurs is a riffle. Sediments in riffle areas tend to be more coarse and heterogeneous. Pools also form at bends in the stream channel as water flows in a helical pattern undercutting the bank. A pool forms where the tumbling water scours out the bottom. Sediment eroded from this area is deposited on the opposite bank forming bars and possibly islands in larger rivers. Bars usually make poor habitats for bottom dwelling organisms because of their unstable, shifting
Figure 4. Pool and bar formation at bends in a stream. nature.
Sources of Energy Input Woodland streams normally have headwaters that are shallow, narrow, and heavily shaded. Stream productivity is strongly influenced by riparian vegetation which reduces autotrophic production, and contributes large amounts of allochthonous detritus that are colonized by bacteria and fungi. As this low order productivity is less than respiration (P
The River Continuum Concept Physical, chemical, and biological characteristics of a river change from headwaters to its mouth. The RCC is a holistic view of river morphology, biotic assemblages, and ecosystem parameters that describes consistent, predictable changes in freshwater habitats and trophic organization along a streams course. It emphasizes that the lotic ecosystem includes the entire watershed- the channel itself, the riparian zone, and the upslope drainages. Viewing the river as a continuum, it predicts that downstream biotic communities are tightly coupled with upstream processes, such as detrital processing, FPOM transport, and upstream disturbances. The RCC was developed in the eastern U. S. and is a typical of forested eastern rivers and may not reflect conditions elsewhere, including the western U.S., where channel geomorphology and biotic assemblages might differ considerably. Woodland streams normally have headwaters in which the basin is narrow and heavily shaded, with the water cool and shallow. Productivity is strongly influenced by riparian vegetation which reduces autotrophic production, and contributes large amounts of allochthonous detritus (P Parameter Typical headwaters Typical mid-reaches Typical river mouths gradient high intermediate to low low to very low dominant substrate coarse (bedrock, intermediate (small fine (sand, silt) size/type boulders, big cobbles) cobbles, gravels) water turbulence high intermediate to low low turbidity low intermediate high relative water temp. cold intermediate warm DO high intermediate low channel profile shallow, narrow intermediate deep, wide canopy coverage closed open none shredders and collectors collectors- gatherers and (gatherers in collectors (as in filterers in the benthos dominant FFGs depositional habitats headwaters) and and filterers on hard and filterers in erosional scrapers substrates, e.g. exposed habitats) rocks and woody debris allochthonus CPOM, autochthonus CPOM, downstream transport of primary C inputs allochthonus CPOM and downstream FPOM FPOM and DOM transport < 1; periphyton and > 1; periphyton and < 1; deeper portions of P/R ratio vascular hydrophytes vascular hydrophytes the channel frequently scarce relatively abundant aphotic response to rapid intermediate Slow disturbance Table 1. Summary of River Continuum Concepts