DESCRIPTION
2.12 Division Dam and Fishway This is the very upstream end of the project reach. This dam is owned and operated by the Corps of Engineers Mill Creek Flood Control Project. The purpose of the dam is to divert flow from Mill Creek into the Yellowhawk/Garrison canal and subsequently to Yellowhawk and Garrison Creeks for irrigation. The site was surveyed for elevations and fishway and dam layout. The concrete sill (Reach 1) downstream of the dam is about 0.3 to 0.4 feet below the dam apron. The site was analyzed for two conditions per the CORP operating criteria (6" and 18" fishway exit slot width). Additional diversion structures are located within Yellowhawk Creek but are not part of this assessment. The hydraulic analysis is for up to 400 cfs with gates closed. Above that, gates open and fish passage is good. The description of the dam and fishway and their operation is taken primarily from Corps' Biological Assessment for Operation and Maintenance of the Mill Creek Flood Control Project, dated March 2003 and from observations and survey measurements made by the authors. The Division Dam includes four bulkhead gates, a fish ladder, and diversion headworks. Each bulkhead gate is 25 feet wide by 2 feet high. When the arm gates are closed, they create a dam two feet high to divert water through the headworks. Additional flow passes over the gates and through the fish ladder. The gates can be raised above the bridge deck for flood operations. The clear opening through the div
2.12 Division Dam and Fishway
This is the very upstream end of the project reach. This dam is owned and operated by the Corps
of Engineers Mill Creek Flood Control Project. The purpose of the dam is to
divert flow from Mill Creek into the Yellowhawk/Garrison canal and subsequently
to Yellowhawk and Garrison Creeks for irrigation. The site was surveyed for elevations
and fishway and dam layout. The concrete sill (Reach 1) downstream of the dam
is about 0.3 to 0.4 feet below the dam apron. The site was analyzed for two
conditions per the CORP operating criteria (6" and 18" fishway exit slot
width). Additional diversion structures
are located within Yellowhawk Creek but are not part of this assessment. The
hydraulic analysis is for up to 400 cfs with gates closed. Above that, gates
open and fish passage is good. The
description of the dam and fishway and their operation is taken primarily from
Corps' Biological Assessment for Operation and Maintenance of the Mill Creek
Flood Control Project, dated March 2003 and from observations and survey
measurements made by the authors. The Division Dam includes four bulkhead
gates, a fish ladder, and diversion headworks. Each bulkhead gate is 25 feet
wide by 2 feet high. When the arm gates are closed, they create a dam two feet
high to divert water through the headworks. Additional flow passes over the
gates and through the fish ladder. The gates can be raised above the bridge
deck for flood operations. The clear opening through the division works when
the arm gates are open is 96 feet wide by 6 feet high.
Fish have unobstructed passage under the dam gates when the
gates are open though the downstream dam apron is shallow and may block fish at
some low flows. The diversion headworks is a concrete structure with three
bays. The center bay includes a 14-foot-wide by 6-foot-high radial gate. The
right bay is covered by needle gates (a series of vertical planks), except for
a 16-inch-wide slot to allow for fish passage from Yellowhawk Creek to Mill
Creek. The bottom of the slot is at the same elevation as the stream bottom
(upstream) and concrete apron (downstream). With head on the slot from 1 to 3
feet, the corresponding water velocities are about 8 to 14 fps. Under current
operations, this slot is always open. The left headworks bay is completely
sealed off with needle gates.
During flood events, the Corps assumes control of water
releases and/or diversions in order to regulate flows in a manner that is
optimal for flood protection. The Washington Department of Ecology is
responsible for flow regulation when the flows are below flood diversion
criteria. The WDOE Water Master directs the amount of water diverted from Mill Creek
into the Yellowhawk/Garrison canal for the purposes of satisfying water rights
and maintaining adequate flows for fish and related habitat.
