FLOOD10415

• County: Conejos
• Stream Name: Alamosa River
• Title: Assessment Report on the Alamosa River, Between Terrace Reservior and Gunbarrel Road
• Date: 10/1/2002
• Prepared For: Alamosa River Watershed Restoration Project
• Prepared By: Black Creek Hydrology, LLC
• Floodplain - Doc Type: Flood Mitigation/Flood Warning/Watershed Restoration

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purposes, the bridge at Forest Route road FR250 is 11,800 feet downstream of the Terrace Reservoir dam, the blidge on

County Road BB is at 21,840 feet and the bridge at County Road Z is 43,080 below the dam. The section of river

between about 46,300 feet and 53,500 feet (about 1.4 miles) was not walked or assessed and was omitted from the

study.

Assessment Findings

The liver upstream of GPS point number 43 generally has a steeper gradient than downstream of point number 43 and,

depending on slope, would lik~ly be classifIed as a stream type of C3c (Rosgen 1996). With the exception of a few

localized problems, it seems tq have reasonably good sediment transport competence with generally stable banks and

bed. It contains a few depositional features including lateral and mid-channel bars (see photographs IP14GPS7 and

IP23GPSI5). Stability problems (some severe) increase for a short distance downstream of point number 15 but are

low from about point 32 to POlnt 43. The aerial photos (Appendix B) indicating points 41, 41 and 43 show channel

characteristics typical of the reach from 32 to 43.

A dramatic change in channel ,condition occurs downstream of point 43 and can be easily seen on the aerial photograph

I
showing points 44 to 48. That reach appears to be either a localized transition or disturbed area that influences the

spatial and temporal sediment transport in the river. For one reason or another (perhaps change in gradient, historic

catastrophic flow event, change in width/depth ratio or man or animal impacts) the sediment transport competence

rapidly declines at his location and a large, unstable depositional feature exists in that area. Consequently, the section

of channel is highly sinuous, has steep banks and rapidly migrates laterally. There is evidence of historic meander

oxbow formation. A new cuto:tf-channel and oxbow will likely form in the reach during next year or two. The

unstable condition exists for about 2,000 feet before the channel develops a more normal sinuosity. It is possibh: that

much of the depositional material accumulating in this reach was derived from bank erosion downstream of point 15.

There are several locations along the assessment reach that contain similar, though usually less prominent, depositional

features. While these features 'are depositional, they are also sources of in-channel sediment supply since they often

have high, eroding banks. Deposition may be temporary since materials that accumulate in the features are eventually

be eroded and mobilized for conveyance to other downstream locations.

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