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<br />Reach 4 (RM 106-130) Just above Aneth, UT to below the "Mixer."
<br />
<br />The valley width decreases slightly from Reach 3 to Reach 4. Total water surface of this reach is
<br />less than the adjacent reaches over a range of flows, indicating a slightly narrower and deeper main
<br />channel in Reach 4. Many of the rest of the characteristics of this reach are intermediate between
<br />Reach 3 and 5, including both channel gradient (0.00164 ft/ft) and sinuosity, both of which are
<br />increasing in an upstream direction from Reach 3. Reach 4 has a large amount of island area, second
<br />only to Reach 5, indicating the presence of extensive secondary channels. Many of the islands
<br />located in this reach are not extensively vegetated and are fairly unstable, especially when compared
<br />to islands in Reach 5, but more stable than islands in Reach 3. Low velocity habitats such as
<br />slackwaters, pools, eddies, and backwaters are more prevalent in this reach than, in those
<br />downstream. Several large permanent backwaters are located near the confluence of the Mancos
<br />River. No municipalities occur near the river in this reach, reducing effects of human activity
<br />relative to the adjacent reaches. One perennial tributary, the Mancos River, and three major
<br />intermittent tributaries join the river within this reach.
<br />
<br />Reach 5 (RM 131-154) Below the "Mixer" to above Shiprock, NM.
<br />
<br />Valley width in this reach is greatest of all eight geomorphic reaches. Channel gradient (0.00193
<br />ft/ft) and sinuosity are intermediate between the two adjacent reaches, both increasing in an upstream
<br />direction. Total water surface area is relatively large in the reach at most flow levels (largest at high
<br />flows) as a result of a broad, shallow main channel and extensive overbank flooding at high flows.
<br />Island area is largest in this reach at all flow levels, indicating extensive secondary channels. Many
<br />of the islands in this reach are very well vegetated with tamarisk and remnant cottonwood stands and
<br />are relatively stable compared to islands in other reaches. Total amount of low velocity habitat in
<br />this reach is probably greater than most other reaches, although density of low velocity habitat may
<br />be higher in isolated areas of other reaches, particularly Reach 8. Cobble and mixed substrates are
<br />most prevalent in the reach. Reach 5 includes municipality of Shiprock, NM, and is substantially
<br />effected by human activity, including a large increase in irrigated land adjacent to the river and the
<br />presence ofa major diversion structure (Cudie Diversion). One perennial tributary, the Chaco River,
<br />and two major ephemeral tributaries enter the river in this reach.
<br />
<br />Reach 6 (RM 155-180) Shiprock, NM to just below the confluence of the Animas River.
<br />
<br />Valley width in this reach decreases slightly from Reach 5, but is still second greatest of all reaches.
<br />Average channel gradient (0.00209 ft/ft) and sinuosity in the reach are also second highest of all
<br />reaches. Island area in this reach is significantly less than in Reach 5, indicating less extensive
<br />secondary channel development. Cobble substrate is prevalent in the reach. The reach includes the
<br />municipalities of Fruitland, Kirkland, and Farmington, NM, and may be the most effected by human
<br />activity, with irrigated land adjacent to 100 percent of the river and four major diversion dams
<br />located within the reach. Three perennial tributaries enter the San Juan River in this reach, the La
<br />Plata River, Ojo Amarillo Canyon, and the Animas River. Ojo Amarillo Canyon, historically an
<br />ephemeral wash, is now more perennial in character primarily due to irrigation return flows from
<br />relatively recent agricultural development in the area.
<br />
<br />San Juan River Recovery Implementation Program
<br />Summary Report
<br />
<br />7
<br />
<br />17 January 1997
<br />PR-576-2
<br />
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