7884

• UCREFRP Catalog Number: 7884
• Author: Hydrosphere Resource Consultants.
• Title: Reconnaissance Evaluation of Yampa River Diversion Structures.
• USFW Year: 1995.
• USFW - Doc Type: Boulder, CO.
• Copyright Material: NO

Introduction
Diversion Structures
Irrigation diversions from the mainstem of the Yampa River are made using a variety of
structures. However, they can generally be classified as one of three types--fixed or temporary
structures to facilitate gravity diversions, or pumps. The fixed structures are mainly rock dams
that survive intact from year to year. The temporary structures tend to be gravel berms and
wing dams pushed into place using heavy equipment; these structures usually wash out during
the next runoff or significant storm. Pump diversions may or may not include in-stream
excavations or structures to pond or redirect river flows toward intakes.
Irrigation diversions occur primarily between May and September with crop watering
requirements being greatest in July and August. It is also in July that the runoff hydrograph
drops most significantly. As a result, instream activities to maintain and rebuild diversion
structures typically take place in July and early August as irrigators strive to get enough water
to their pumps or headgates. Because these rebuilt structures typically only need to be effective
for a month or two they are mainly temporary in nature; those that are not breached by internal
erosion within a few months are usually washed out the following spring.
Section 404 of the Clean Water Act calls for federal regulation of activities that result in
dredging or filling of streams. However, there is in the legislation itself a broad agricultural
exemption to this requirement that permits irrigators to maintain their diversion facilities
without federal regulation. Section 9 of the Endangered Species Act creates federal regulatory
authority over private activities that "harass or harm" endangered species; however it remains
to be resolved whether habitat modification (such as might occur with diversion structure
maintenance in the lower reaches of the Yampa) constitutes such "harassment". Accordingly,
most of the activities related to creating and maintaining diversion structures along the Yampa
River mainstem have historically been unmonitored and unregulated.
PROBLEM STATEMENT
Because instream maintenance of diversion facilities takes place at about the same time as
Colorado River squawfish are believed to migrate upstream, concerns have been raised about
the effects of diversion structures and the related instream construction activities on the
endangered fish. These concerns relate both to fish passage and to habitat disturbance.
Somewhat lesser but still important concerns relate to the effects diversion structures have on
recreational activities such as boating and fishing and the implications for water rights
administration of increasing the "passibility" of diversion structures.
Diversion structures can impede fish passage by creating drops that are too high for fish
to overcome, by directing river flows through rock and rubble interstices to small for adult fish
to swim through, and by reducing downstream flow depths to levels that do not permit passage.
Porous structures such as rubble dams which block some but not all channels of the river can
also create "false passages", drawing upstream migrating fish away from channels that might
permit passage. These characteristics, as well as construction activities themselves, may result
in potential adverse impacts to the listed fishes.
1-4