North Poudre Sediment Study Final Report

• Project Type: Ag/Muni Grant
• Contract/PO #: C153779
• Applicant: North Poudre Irrigation Company
• Project Name: North Fork Poudre Sedimentation Study
• Title: North Fork Cache la Poudre River Sedimentation Study Final Report
• Date: 1/1/1999
• County: Larimer
• Water Conservation - Doc Type: Final Report

The riffles at all locations downstream from the dam were largely unaffected by

the sediment release. Marginal bars comprised of fine sediment lined the channel banks

along riffles, and isolated deposits were observed in low velocity areas behind cobbles

and boulders comprising the bed of the channel. Fine sediment also infiltrated the

interstices of the coarse substrate, up to 6 cm deep, Because the pools are critical

overwinter habitat for the fish within the North Fork below Halligan, this study focused

on two pools within a 40001 study reach located within The Nature Conservancy's

Phantom Canyon Preserve. The two pools were named the Ouzel Pool and the Tick Pool

(Figure I).

Flushing Flows

After the sedimenl release in September 1996, the discharge from Halligan

Reservoir was dropped to the instream flow requirement of 0,1 ml/s. Between that time

and late February 1997. flows were kepi approximately at the 0,1 m31s discharge. In late

February 1997. a stepped. experimental discharge was released from the outlet valves at

the base of Halligan dam with the intent to flush sediment from the pools and to allow a

repeat survey to assess the quantity and nature of sediment mo\'ement. During the

experimental release. a peak discharge of 3 A m3/s was attained. and then the discharge

was dropped back to the 0.1 m3/s magnitude (Figure 2).

Typically, flushing flows refer to discharges designed to flush sediments of a

particular grain size from riffles and runs to improve spawning habitat (Kondolf and

Wilcock. 1996: \Vilcock. et al. 1996), Most often, the application of flushing discharges

pertains to regulated rivers where tributary inflow introduces abundant tine sediments that

cover spawning gravels, Few studies have investigated the flow requirements of flushing

sediments from pools to enhance over-winter pool volume for fish, Wesche (1991)

evaluated several Wyoming rivers and discharge requiremenls for flushing pools, In this

study. flushing flows refers to the magnitude and duration of flows that would sufficiently

t1ush sediments downstream from Halligan Reservoir to minimize the deleterious effects

to fish from pool intllling. This report does not address the best flushing regime for

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