Larry A. Schock, Civil Engineering Specialist
Water Resources Division, Missoula Regional Office
542-5885 [email protected]
Located 30 miles SW of Darby Mt.
143 foot-high, 800 foot-long Earthen Dam
Constructed in 1939
Owned by the DNRC
Managed by the State Water Projects Bureau
Provides 10,000 AF of late season irrigation water for
over 25,000 acres in the Bitterroot Valley.
Provides 15,000 AF of water for instream flows in the
Bitterroot River for fisheries habitat enhancement.
Contracts for the irrigation water are held by Farmers,
Ranchers, Ditch Companies, and Irrigation Districts
from Conner to Florence.
The irrigation water contracts are managed by the
Painted Rocks Water Users Association (PRWUA).
The PRWUA is a legal entity that is made up of the
members of the agricultural community who use the
water. They hold the first right of refusal for the
irrigation water contracts.
Contracts for the fisheries water are held by the
Montana Department of Fish Wildlife and Parks (DFWP).
The fisheries water contracts are managed by the
DFWP, and are intended to insure the viability and
health of the Bitterroot River.
The primary goal of the fisheries water is to insure that
the Bitterroot River does not go dry at Bell Crossing. A
secondary goal is to help reduce water temperatures in
the West Fork and upper Bitterroot Rivers.
Up until 1985 the Bitterroot River used to go dry at Bell
Crossing during dry years, and was so low during
normal years that it adversely impacted fish survivability.
The fisheries water contracts allows the DFWP to
manage the river in order to insure that there is water in
the Bitterroot River at Bell Crossing.
The targeted flows at Bell Crossing are 450-600 cfs
during normal years and 200-250 cfs during dry years.
The water right for Painted Rocks Dam allows the
reservoir to be filled during the year, but only allows for
water deliveries from May 1st thru September 30th.
The reservoir is primarily filled during the early portion
of the spring run-off, prior to the peak run-off flows,
during the months of March thru May.
Painted Rocks Reservoir is not a flood control
structure, it is a storage reservoir that is slowly filled in
a regulated manner in order to insure the safety of the
dam.
The reservoir is managed so that it fills and spills by
Memorial Day weekend in order to insure a full pool for
recreational purposes.
Once run-off is over and the reservoir is full, it is held in
a state of equilibrium (outflows = inflows).
Once the reservoir is full, very little water flows over
the spillway, and the majority of the water is released
thru the low level outlet to insure cold water in the
river.
While the DNRC actively manages the dam and reservoir
operations, we do not determine when the contract water
is released.
Contract water releases are determined by the PRWUA,
the DFWP, and the Bitterroot River Water Commissioner.
In order to minimize impacts on the river, gate
adjustments are made in the 50-75 cfs range, whenever
possible. Contract Water deliveries will be released in
addition to the inflows (outflows = inflows + contract
deliveries).
Under Montana State Law, Painted Rocks Dam is
classified as a “High Hazard” dam, and it must be
regulated and managed to higher standard than most
dams.
The “High Hazard” classification is not a reflection of
the condition of the dam, it is an indication of the
hazards it would represent if it were to fail.
In the very unlikely event that there was a catastrophic
failure of the dam it would cause wide spread flooding
in the valley.
When it comes to the management of Painted Rocks
Dam, the DNRC’s #1 priority is the public’s safety and
welfare.
The DNRC works closely with the local authorities to
insure that the Emergency Action Plan for the dam is
constantly being updated and everyone understands
what is needed in the event of some sort of emergency.
Since the reservoir is only used for seasonal storage, it
is only full 3 to 4 months out of the year, so the hazard
represented by the dam is less than it would be if the
reservoir was full year round.
The monitoring and management of Painted Rocks Dam
far exceeds the requirements of the Montana Dam Safety
regulations.
When the reservoir is filling, visits are normally made to
the dam twice a week, thru the end of the spring run-off
event.
When the reservoir is full and spilling, visits are made
to the dam at least once a week, often times twice a
week.
Once contract water deliveries begin in July visits are
made to the dam at least once a week, often times
twice a week.
On average, once contract water deliveries begin the
reservoir level will drop approximately ½ ft. per day
during July and approximately 1 ft. per day during
August and September.
By September 30th the reservoir level is down about 58
ft., with about 7,350 acre-feet of storage, which
represents only about 42 ft. of water behind the dam.
Due to the greatly reduced level of the reservoir from
October thru February it is very unlikely that a dam
failure could occur.
Larry A. Schock, Civil Engineering Specialist
Water Resources Division, Missoula Regional Office
542-5885 [email protected]