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

At the spawning bar the discharge at incipient motion is slightly lower, and ranged from 17,000 to
21,000 cfs and 19,500 to 30,000 cfs for subsurface DSO's of 28mm and 42mm respectively. For both sites the
difference in discharge for incipient motion for surface particles ranging in size from the subsurface D50 to
160mm is relatively small. This is consistent with the equal mobility concept discussed by both Andrews
(1983) and Parker, et al. (1982) which accounts for the effects of protrusion and hiding in a coarse gravel
bed system (see Section 2.4 and Appendix A).

The National Park Service study concluded that the incipient motion of the median bed material
particle (75 mm) would occur at about 21,500 cfs, which was considered to have a return period of about 20
years (O'Brien, 1984, pages 26 and 42). Using a more comprehensive analysis procedure and additional
data, results presented herein indicate that a 75 mm particle would be at incipient motion in the range from
17,500 to 26,500 cfs for Mathers Hole and from 19,500 to 27,500 cfs for the spawning bar using subsurface
particle sizes ranging from 28 mm to 72 mm. Given that measured data were used in all aspects of the
analysis, the results reported here are considered reasonable and indicate that significant movement of the
cobbles on the cobble bars in Yampa Canyon is a relatively rare event.

As previously discussed, incipient motion is a threshold between no motion and motion. The lower
incipient motion curve is the minimum discharge required to initiate motion of a few particles.
Consequently, at incipient motion wholesale movement the gravel bars is not expected. Rather, a much
higher flow would be required to significantly alter the gravel bars. This underscores the conclusion that a
significant movement of the cobble bars in Yampa Canyon is a relatively rare event.

5.5 Conclusions

The concept of equilibrium has its roots in the analysis of purely alluvial streams, such as sand bed
streams in low gradient areas, and implies that the channel boundary has adjusted so that transport capacity
provided by the channel equals the sediment supply provided to the channel. Any natural or man-made
impact that alters this balance, or equilibrium, will result in channel response and modification so that over a
period of years, the transport capacity again equals the sediment supply. Given that the Yampa River
Canyon is thought to be supply limited for most of the sand size fractions, the channel likely has excess
transport capacity. Therefore, it is not appropriate to use equilibrium when describing the result that no
progressive aggradation or degradation is occurring in the canyon. While this distinction is subtle and may
seem trivial, the term equilibrium has the connotation of channel stability that will be disrupted by any
disturbance. This is not necessarily true in a supply limited system.

With the exception of flows less than 2,000 cfs at Deerlodge Park, neither the sediment data at
Deerlodge Park nor at Mathers Hole indicate that there is a distinguishable relationship between sediment
discharge and water discharge. Generally the data varies over nearly two orders of magnitude for a given
discharge. Furthermore, comparison of the raw data at the two sites also reveals that there is no significant
difference between the data at Mathers Hole or at Deerlodge Park. From this, it is concluded that there is
only a weak relationship between water and sediment discharge at both Deerlodge Park and Mathers Hole.
Such a weak relationship is indicative of supply limited streams where sediment transport depends to a
greater degree on the watershed supply and other hydrologic factors than on re-entrainment of sediments
derived from the bed or banks of the channel.

Bias corrected rating curves have been developed for use in subsequent analysis. These rating
curves were used to develop effective discharge estimates for Lily, Deerlodge Park, and Mathers Hole. The
effective discharge (the increment of discharge that transports the largest fraction of the annual sediment
load over a period of years) is identical at Deerlodge Park and Mathers Hole. This observation compared
with a sediment continuity analysis leads to the conclusion that the reach between Deerlodge Park and
Mathers Hole cannot be shown to be either progressively aggrading or degrading. The canyon bound reach
of the Yampa River between Deerlodge Park and Mathers Hole is a supply limited channel. Under current
conditions, sediment deliveries from Deerlodge Park are readily transported through the Canyon. The
above discussion pertains solely to the overall tendencies of the study reach. However, it is probable that

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