Rivers and Streams, Characteristics - Hydro-Environmental Analysis: Freshwater Environments - page 37

2.2.3 r oSGen c LaSSIfIcatIon
A variety of classiication systems has been designed to aid in assessing the relationships between
channels, basin geomorphology, and watershed conditions, several of which are summarized in
the NRCS National Engineering Handbook (Part 654, NRCS 2007). One of the more commonly
used classiication systems to identify the current status of channel reaches (rather than the entire
system) for rivers is that developed by Rosgen (1994, 1996; Rosgen and Silvey 1996; NRCS 2007).
The Rosgen scheme includes four levels of detail, ranging from qualitative descriptions to detailed
qualitative comparisons, each with a successively more detailed or iner deinition of the dimension,
pattern, and proile of the stream reach being classiied (NRCS 2007).
The irst level of the scheme uses information on channel slopes and channel patterns to subdi-
vide the rivers or streams into a series of broad stream types (“A” through “H”; Figures 2.25 and
2.26). Each of these types for North Carolina streams is illustrated in Figure 2.27 from Harman and
Jennings (1999). The distinction between the stream types is made based on whether the streams
are single-thread or multithread channels and on four morphological parameters: entrenchment
ratio, width to depth ratio, sinuosity, and slope. The entrenchment ratio is a metric used to deine
the extent of incision, and it is estimated from the ratio of the lood-prone width (generally twice
the elevation of the bank-full depth) to the bank-full width. The width to depth ratio is the bank-full
width divided by the mean bank-full depth. The break between single-channel classiications is 12,
meaning that the bank-full width is 12 times greater than the mean bank-full depth (Figure 2.26).
The sinuosity is estimated by the channel length divided by a straight-line valley length. The water-
surface slope is estimated from the top of a rifle to the top of another rifle at least 20 bank-full
widths downstream (Harman and Jennings 1999). With the appropriate information, this level of
classiication can be determined using aerial photography and topographic information. The infor-
mation needed is as listed by the U.S. Environmental Protection Agency (EPA) in the Watershed
Academy training module on the Rosgen system. The Level I stream classiication serves four
primary functions:
1. Provides for the initial integration of basin characteristics, valley types, and landforms
with stream system morphology.
2. Provides for a consistent initial framework for organizing river information and communi-
cating the aspects of river morphology. The mapping of physiographic attributes at Level I
can quickly determine the location and the approximate percentage of river types within a
watershed and valley type.
3. Assists in the setting of priorities for conducting more detailed assessments and compan-
ion inventories.
4. Correlates similar general level inventories such as isheries habitats, river boating catego-
ries, and riparian habitats with companion river inventories.
The second level of classiication (Level II stream-type delineation) is based on the characteris-
tics of the channel cross section, including the D50 or median particle size (see Figure 2.26). This
classiication also includes longitudinal proile measurements such as the slope and bed features
(coniguration of rifles/pools, rapids, steps/pools, etc.) and planform (pattern) measurements (such
as sinuosity and meander width ratios).
The third level of Rosgen stream classiication (Level III) is the stream state or condition assess-
ment. This is an assessment and prediction of the stream's condition and its stability and requires an
assessment of the channel erosion, riparian condition, channel modiication, and other characteris-
tics. The fourth level is a veriication of the predictions made in Level III (Rosgen and Silvey 1996).
While Levels I and II are used to describe the present condition of a channel, Levels III and IV are
used to evaluate and validate an assessment of the stream's condition and its departure from the
optimum or potential condition (NRCS 2007).