Fundamentals of Fluvial Geomorphology and Channel Processes
In other conditions, such as in high mountain stream flowing in very coarse glacially deposited
materials or significantly controlled by fallen timber would suggest a non-alluvial system.
Alluvial channels may also be classified as either perennial, intermittent, or ephemeral.
A perennial stream is one which has flow at all times. An intermittent stream has the
potential for continued flow, but at times the entire flow is absorbed by the bed material. This
may be seasonal in nature. An ephemeral stream only has flow following a rainfall event.
When carrying flow, intermittent and ephemeral streams both have characteristics very similar
to perennial streams.
Another classification methodology by Schumm (1977) includes consideration of the
type of sediment load being transported by the stream, the percentage of silt and clay in the
channel bed and banks, and the stability of the channel. Sediment load refers to the type or
size of material being transported by a stream. The total load can be divided into the bed
sediment load and the wash load. The bed sediment load is composed of particles of a size
found in appreciable quantities in the bed of the stream. The wash load is composed of those
finer particles that are found in small quantities in the shifting portions of the bed. Frequently,
the sediment load is divided into the bed load, those particles moving on or near the bed, and
the suspended load, those particles moving in the water column. The size of particles
moving as suspended load may include a portion of the bed sediment load, depending on the
energy available for transport (ASCE, 1977). For example, the suspended load frequently
reported by U.S. Geological Survey publications usually includes a portion of the bed
sediment load and all of the wash load. Sediment discharge is the rate at which the sediment
load is being supplied or transported through a reach.
For purposes of this classification system, a stable channel complies with Mackin's
definition of a graded stream. An unstable stream may be either degrading (eroding) or
aggrading (depositing). In the context of the definition of a graded stream being in balance
between sediment supplied and sediment transported, an aggrading stream has excess
sediment supply and a degrading stream has a deficit of sediment supply.
Table 2.2 presents a summary of this classification system and describes the response
of the river segment to instability and a description of the stable segment. It is very important
to note that the work on which this classification was based was conducted in the Midwestern
U.S.; therefore, the classification system represents an interpretation of empirical data.
Extrapolation of the classification beyond the data base should be done cautiously.
Schumm and Meyer (1979) presented the channel classification shown in Figure 2.12,
which is based on channel planform, sediment load, energy, and relative stability. As with any
classification system, Figure 2.12 implies that river segments can be conveniently subdivided
into clearly discernable groups. In reality, a continuum of channel types exists and the
application of the classification system requires judgement.