Rivers and Streams, Characteristics - Hydro-Environmental Analysis: Freshwater Environments - page 29

would be expected to have a steep gradient with rifles, rapids, and falls. The sunlight reaching
these streams is limited due to the shading of the riparian forest, so photosynthesis is limited and
heterotrophs (the consumers) as opposed to autotrophs (the primary producers) dominate. The pri-
mary source of carbon and energy then comes primarily from outside of the system ( allocthonous
sources, or from outside the stream), such as from falling leaves. Insects are dominated by those that
can shred the course carbonaceous materials, collectors, or predators. Water is cooled by springs
and often supports trout.
In the mid-reaches, the river gradient decreases, there are fewer rapids and falls, and the stream
is wider. The sunlight reaches the water, the stream is warmer, and primary production increases,
so that much of the energy is produced by the autotrophs ( autochthonous sources, or from inside
the stream). Insects feed on the living (or dead) plants. The larger stream also supports a greater
diversity of invertebrates and ish.
As the stream approaches the lower regions, it becomes larger (Orders VII-XII), the gradient
lessens, the temperature increases, and the low increases (due to the larger contributing watershed).
The stream is generally more turbid, reducing light penetration, and energy is supplied by upstream
sources. The primary producers are often dominated by drifting phytoplankton. Backwaters may
exist where turbidity has settled and aquatic plants are abundant. Fish species are omnivores and
plankton feeders such as carp, buffalo, suckers, and paddleish (FISRWG 1998).
The RCC is not universally applicable. However, it does serve as a useful conceptual model and has
stimulated a considerable amount of research since it was irst introduced in 1980 (FISRWG 1998).
2.2.2 S treaM c orrIdor : L ateraL V Iew
Stream characteristics vary laterally (and vertically) as well as longitudinally. This section will dis-
cuss some of those characteristics in the stream channel, in the loodplain, and in the upland areas,
as bordered by the upland fringe.
2.2.2.1 Stream Channel
The stream channel is that portion of the stream that normally conveys water. Moderate lows are
contained within the stream channel and a bank-full discharge is one that ills the entire channel's
cross-sectional area, above which the low spills onto the loodplain.
The stream channel is typically characterized by lateral variations in the cross-sectional shape,
width, depth, bank, and bottom characteristics, as well as longitudinally by its slope, sinuosity, pool
and rifle spacings, etc. These physical characteristics impact and relect the stream's hydraulics
and sediment characteristics, as well as its ecological characteristics. For example, wide and shal-
low systems will have different characteristics from those that are narrow and deep. The width to
depth ratio (the bank-full width divided by the mean bank-full depth) is used as one of the four
morphological characteristics employed to delineate stream types, as discussed in Section 2.2.3 on
the Rosgen classiication scheme (others are the slope, sinuosity, and entrenchment ratio). Another
channel characteristic used in the Rosgen classiication is the median particle size.
The channel characteristics are rarely constant, and are most commonly in a state of lux. This
could be natural or due to anthropogenic impacts. For example, destabilizing streams such as in a
typical incising channel where the streambed degrades until the critical bank height is exceeded and
the bank fails, which increases the channel width and sediment load (Fischenich and Morrow 2000).
2.2.2.1.1 Streamlow and Groundwater Interactions
Not all rivers continuously low and streams and rivers can be classiied based on their duration and
frequency of low. Based on their lows, rivers may be classiied as perennial or permanent, inter-
mittent, or ephemeral. Even in perennial rivers, lows vary over time, such as within and between
seasons. The determination of whether a stream is perennial or not has regulatory implications, and
various states may deine perennial as part of their statutes. For example, North Carolina (2003)