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

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

channel with relatively larger bed materials that are more stable than the overall trend of that section. These locations

can be observed as steps in the graph and typically coincide with coarser cobble and small boulder materials observed

in the photographs.

The reach between GPS points 81 and 96 contains relatively low amounts of eroding bank. However, the stream reach

between 81 and 86 appeared to be a depositional area with its own inherent instability. Between GPS points 90 and 96

the channel appeared to be steeper, contains coarser cobble material and is quite stable with little or no eroding bank.

Downstream of GPS point 96,. and through the remainder of the assessment reach, the channel contains substantial

eroding banks and some of the most unstable channel observed during the assessment.

Discussion

The large quantity of eroding bank indicates a large potential input of sediment into the river system. It is understood

that not all banks will erode atthe same rate. Some of the observed banks likely erode at less than a foot a year while

others have been observed by local residents to erode at rates of 5, 10 and even 20 feet per year. Speculatively, at an

I
average annual bank loss rate 9f one foot per year from the documented eroding banks, there would be an annual influx

I
of about 1,300 cubic yards of sediment into river system. That would be enough material to fill to bankfull capacity a

300 foot long section of the channel each year.

There are three monitoring cross sections in the assessment reach. They are near GPS point 98, GPS point 112 and

immediately upstream of the EI Viejo Ditch headgate. While a re-survey of the cross sections has not been performed

in the past year, the cross sectipns near points 98 and 112 have lost approximately 15 to 20 feet of stream bank.

The affects of excess sediment load may include increased maintenance to irrigation ditches and head gate facilities,
I

locally increased risk of flooding due to deposition and channel aggradation and additional bank erosion. Since the

assessment reach contains numerous points of instability, there are numerous opportunities for bedload materials to

accumulate and redirect flow against the river banks, stress those banks and create additional sources of sediment.

4