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• • ~ ar P l ~o~cP....__ .va4 ••.o ;`. GAIL May 3, 2001 N'•. WEBB ~ o '" o TT '. :; P To: Jim Mimiaga FoF..~o~oP Cortez Journal 37 E. Main St. !~ Commission Expires 7/6!2003 Cortez, CO 81321 A word of caution! As a visitor to the Dolores River Valley I have enjoyed the riparian habitat that enhances the scenic beauty of the area. It was easy to understand why this route was included in the designation of the San Juan Scenic Byway. That is why it is so disappointing to see the valley bottom lands overwhelmed with gravel pits. The rectangular, steep-banked pits remind an observer of a flood plain covered with sewage lagoons. These pits, and particularly those planned near the town of Dolores, not only detract from the beauty of the area but are potentially dangerous. High gradient rivers with large catchment areas are flood producers. As a professional geologist, I look at the Dolores River valley upstream from the town and I see an alluvial flood plain with evidence that the river has repeatedly changed its course across the valley. The stream is gravel-bedded on the alluvial floor and these same gravels are being mined. These gravels that were moved by the river in the past are also used for dikes between the gravel pits and the river. This means the gravel dikes are unstable and will fail. Rivers always try to take the path of least resistance and what could be easier than to exploit a hole in the flood plain? Numerous examples exist of flooding rivers diverted by gravel pits into unexpected new courses that have been very costly to man. This behavior of rivers is so well known I am surprised it appears to have been ignored in the Dolores Vally. Recent floods in the area have been relatively minor events compared to recorded floods in the past. Even the 10,000 cfs flood of 1912, almost twice the discharge of the memorable flood of 1995, pales when compared to the geologic evidence of earlier floods that were 3 to 5 times larger. My main point is that Dolores should expect large flood events and the gravel pits on the flood plain will cause the channel to shift. It should be noted that although dikes along a river can temporarity confine floodwaters, they provide little insurance against movement of the channel itself. This is particularly true if the dikes are built of rocks which the river is competent to move. Furthermore, dikes tend to flush lower flood flows downstream and accentuate bank erosion in downstream reaches. Several houses in the vicinity of the newly proposed gravel pit above Spruce Water Canyon are highly susceptible to flooding from any channel shift induced by the pit and from proposed river confinement. Likewise, channel shift above Dolores has the potential of diverting the main flow of the river through the downtown area. At very least, it is possible to guarantee that when the diversions occur the people of the area will be strongly impacted and it is even possible the Scenic Byway, Highway 145, will ~~