BOARD00168 ‎(2)‎

• Board Meeting Date: 5/22/2007
• Description: ISF Section - ISF Rule 7 - Inundation of ISF Rights
• Board Meetings - Doc Type: Memo

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increased rates of erosion directly below the dam. Because dams often serve to reduce the size of chaIUlel-defining
peak flows, the channel structure downstream of dams also may be affected by their construction.
In contrast to the unidirectional flows of the stream. reservoirs and lakes are dominated by vertical mixing driven
by the wind. This mixing can return small particles suspended in the water column to the surface a number of
times before they settle to the bottom. Consequently, reservoir dynamics are often dictated more by the wind-
driven cycling of these materials within the mixed layer than by the horizontal flux of materials through the lake,
If the reservoir is deep enough, it may become stratified with a warm, well-mixed upper layer overlying a cooler,
lower layer. Because the lower layer is relatively isolated for large periods of time, decomposition can strip
oxygen from the water, producing large regions that are unsuitable for fish,

The increased surface area and decreased shading of reservoirs often means that they are wanner than the stream
they replaced. This is particularly true if they are too shallow to stratify, Often, though not always, this means that
the reach is no longer suitable for coldwater fish species. Many of the aquatic insects that lived on the stream
bottom and relied on current to deliver their food are replaced by organisms, primarily zooplankton, that are better
suited to foraging in the water column or organisms, like worms, that can survive in the fine sediment. Depending
on the size of the impoundment, it may become better suited: to supporting non-native species like pike and bass,
which often prey upon or compete with native cutthroat trout. In short, the transfonnation from a stream to a lake
ecosystem is typically accompanied by dramatic changes in the species present.

Finally, dams can serve as a physical barrier for the movement offish. While the upstream migration of
anadromous fish, like salmon and steelhead trout, from the sea to headwater spawning sites is the most dramatic
example offish movement, many freshwater fish, including Colorado's native cutthroat trout species, move
significant distances during the course of their life cycle. Dams can block fish as they migrate upstream to
spawning habitats in small tributaries and downstream to over-wintering habitats in larger streams and rivers. By .
severing these migration pathways, dams can have significant, deleterious impacts on many fish populations and
downstream sport fisheries.

In summary, inundation transfonns free-flowing streams supporting cold-water species to lakes which potentially
support a different suite of organisms. Certainly, not all the effects outlined above will be realized in every case.
Nonetheless it is imperative that the very real damages resulting from inundation be recognized in order to ensure
full protection of the CWCB's ISF rights, At the very least, upderstanding the impacts produced by inundating a
stream reach will allow the CWCB to determine the appropriate mitigation and compensation strategy for
offsetting the damages to the ISF right.

Thank you for considering my comments, I will be at your May meeting to answer questions you may have.

Sincerely
/Z' ..'
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. John Roach, Ph.D.
Aquatics Specialist

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