9568

• UCREFRP Catalog Number: 9568
• Author: Resource Consultants Inc.
• Title: Sediment Transport Studies of the Little Snake, Yampa, and Green River Systems.
• USFW Year: 1991.
• USFW - Doc Type: Fort Collins, CO.
• Copyright Material: NO

Griffiths (1980) observed that in many mountain streams the supply of gravel that is transported as
bed load is predominantly influenced by the stability of a restricted area of riparian land. During storms,
this zone contn'butes sediment to the stream through random mass movements and stream bank collapses
with the result that floods having similar hydrographs will produce different bed load yields.

In general, it can be stated that supply-limited conditions of sediment transport prevail when the
amount of material of a given size range in transport is limited by its availability and not by the competence
of the flow (Pitlick and Thorne, 1987).

5.1.2 Stable and Equilibrium Channels. The concept of stability, as used above, requires further
clarification. In particular, the concept of an equilibrium or graded stream channel needs to be defined.
The concept of a graded stream has its origins in geology and geomorphology. One of the early and most
comprehensive treatments of the concept of equilibrium or graded streams is the classic paper by Mackin in
1948. Schumm (1977), citing this reference defines a stable, or in equilibrium channel as one with a balance
between its transporting capacity and the amount of material supplied to it (pg. 9). Note that this classic
concept of equilibrium assumes alluvial channel conditions, that is a mobile channel boundary that is free to
adjust and to develop the geometry necessary to balance transport capacity with sediment supply. As such,
this definition of equilibrium is not appropriate in describing stable conditions (i.e. neither significant
erosion or deposition over time) in supply limited streams, where there is excess transport capacity.

As an illustration of this subtle but important distinction, consider the two extremes defined by a
purely sand bed stream in an arid region, and a cobblefboulder bed high mountain stream. Over time the
sand bed stream has developed a channel geometry necessary to balance its transport capacity with a
relatively large sediment supply, and based on the classical defmition of equilibrium would be considered a
stable or graded channel. In such a channel, a change in discharge or sediment supply could disrupt the
existing channel stability, leading to some change in channel morphology so that transport capacity and
sediment supply would again be balanced, or equal (i. e., in equilibrium). An important characteristic of the
sand bed stream is that the bed material can be transported, and thus the boundary adjusted, over the entire
range of flows to which the channel is subjected.

In contrast, the bed material in the cobble/boulder bed stream may be mobile only under
infrequent, high flow conditions. Consequently, geomorphically effective events for transporting sediment
and restructuring channels occur infrequently (Grant, Swanson, and Wolman 1990). The sediment that is
transported by the stream is controlled by the supply from the surface erosion of bare slopes, landslides,
debris flows and erosion of the stream bed and banks. Since the input to the steep, cobblefboulder stream
is variable depending on the specific characteristics of the watershed, the transport rate is equally variable
(Whittaker, 1987). Thus, the finer material supplied from watershed sources is effectively supply limited.
The supply limited mountain stream, with comparatively little sediment supply and a large transport
capacity, is very stable, but should not be referred to as "in equilibrium" given that the transport capacity and
sediment supply are not in equilibrium, or in balance. Furthermore, in a supply limited channel, some
increase in sediment supply, or alternatively, some decrease in transport capacity, could be tolerated without
altering the overall morphology of the channel.

Clearly, the above two examples represent extremes in a spectrum of channel morphology, from
purely sand bed to cobble and boulder bed. It is possible that a channel reach can be composed of
subreaches with cobble and boulder beds interspersed with sand bed reaches. An example of this type of
channel occurs in steep mountain streams which have an alternating riffle-pool arrangement. In these
channels the riffles are usually composed of large sized material in the cobble to boulder size ranges while
the pools can be composed of fmer material deposited over coarse material. Since the canyon bound reach
of the Yampa river in the study area has this type of pool-riffle arrangement, a qualitative discussion of the
interaction of the flow and sediment is warranted.

When the discharge in the channel is low, the flow alternates from tranquil flow in the pools to
more rapid flow over the crests of the riffles. Therefore, at low discharges, sands and fine sediments can

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