Monday, February 14, 2011

Highway engineering, aquatic ecosystems and a little bit of gold

A second installment of Geology begets biology

This is a view looking east towards the Continental Divide in Summit County Colorado. The highway snaking up the hill is Interstate 70. The highway bottoms out at 8900 feet and then climbs steadily to 11,150 feet where the road enters the Eisenhower Tunnel and makes its way under the Continental Divide to the east slope.

The original plans had the tunnel a few miles west and a couple of thousand feet below its current placement. However, before construction began, the bean counters started to complain about the length of the tunnel and asked if there could be another placement for the bore. So, apparently the powers that were decided that the geology in the mountain should all be somewhat similar and decided to drill the tunnel in its present configuration. So, the highway now climbs up the Straight Creek drainage to the 11,150 foot elevation and then makes its way east.
The problem with this plan is the gradient of the highway exceeds the Interstate standards and requires massive amounts of sand to keep the highway open. It is just too steep on those icy days.

Normally, a high country stream like this is relatively sediment poor and would be able to easily transport any sediment it receives from erosion. However, now as spring arrives and the snow melts, huge amounts of sand are washed into Straight Creek severely impacting water quality. The towns of Dillon CO. and Dillon Valley CO. rely on this watershed for their potable water. In addition, Straight Creek runs into the Blue River with its gold medal fisheries.

In the early 1990's my biology classes spent some quality time in Straight Creek doing pebble counts and macro-invertebrate census collections. Yes, the water was quite cold. Our findings: There were no macro-invertebrates as their habitat had been inundated by feet of sand. The pebbles we found, were all sand grains. No matter our findings, this was a wonderful experience for a group of high school sophomores and our data ended up in someone's Master's thesis.

By now, the Colorado transportation department, local water quality agencies and the state wildlife agency saw the problem and created a series of check dams and sedimentation ponds to catch the sand. In 1993, over 1100 tons of sand were removed from the ponds, about 10% of the total sand applied. Since then, more check dams and sedimentation ponds have been built and by 2000, the majority of sand has been kept from entering the creek. This is an incredible long term maintenance process that in the long run, costs more than the longer tunnel would have back in the 1970's

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