Form No. 4014 07/17Recycled content

Missouri River Flow ManagementBelow Holter Dam

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BackgroundIn recent years the 35-mile tailwater section of the Missouri River downstream from Holter Dam has become the most popular trout fishing river in Montana. The 2015 Montana Fish, Wildlife & Parks statewide fishing survey showed there were 183,000 days of fishing on this river section and anglers spent approximately $60 million on trip-related expenses in 2015 to fish there.

Why is fishing on the Missouri River below Holter Dam so popular? The river below Holter consistently has some of the highest trout populations of any river in Montana (2,000 – 4,000 trout per mile) and the trout are very healthy and large compared to other rivers in the state. This robust trout population is due to highly fertile water, abundant food, and careful management of upstream reservoirs which stabilizes and cools the river downstream from Holter.

How do upstream reservoirs influence flows in the river below Holter dam?Holter is the furthest downstream in a chain of three hydroelectric dams, beginning with Canyon Ferry Dam located near Helena (see map). Canyon Ferry is a federal project completed in 1954 and operated by the U.S. Bureau of Reclamation (Reclamation) primarily for hydropower production, flood control, irrigation, and recreation. Canyon Ferry Reservoir is nearly 10 times larger than Hauser or Holter reservoirs, which are located immediately downstream.

One might think operation of Holter Dam controls flows in the downstream ‘blue ribbon’ river section, but that is not the case. River flow is actually determined by Reclamation’s operation of Canyon Ferry because it stores water during spring runoff and slowly releases the stored water during the rest of the year. As shown in the graph above, the level of Canyon Ferry Reservoir typically declines 12 feet over the fall, winter and early spring months as water is slowly released to make space available for storage of spring runoff. This operational pattern causes reduced outflows during the spring runoff period and enhanced outflows during late summer thru winter as water is released to create space to store the next years’ spring runoff. Note that flow of the Missouri River into Canyon Ferry drops to around 2,000 cubic feet per second during August in an average year, but outflows from Canyon Ferry during August stay near 4,000 cubic feet per second. This enhancement of summertime flows in the river downstream from Canyon Ferry is a major reason why the trout population in the tailwater section below Holter Dam is so healthy.

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Chris H.Hydro Operator/Maintenance Journeyman

13 years of service

Name Operator Year builtReservoir surface

area (acres)

Total volume (acre-feet)

Canyon Ferry

U.S. Bureau of Reclamation

1954 33,500 1,993,000

HauserNorthWestern Energy

1911 3,800 111,000

HolterNorthWestern Energy

1918 4,800 240,000

12000

10000

8000

6000

4000

2000

0

3796

3794

3792

3790

3788

3786

3784

3782

3780

3778

3776Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Flow

(cub

ic ft

per

sec

)

Res

ervo

ir le

vel (

feet

)

In�ow (cfs) Out�ow (cfs) Reservoir level (feet)

Canyon Ferry In�ow, Out�ow & Reservoir Level

Does the operation of Canyon Ferry Dam affect water temperature of the Missouri River in downstream areas?Yes, Canyon Ferry has a major influence on water temperature because it is a deep reservoir and the inlet for the power generators is located approximately 90 feet below the reservoir surface. The deeper waters of lakes and reservoirs are typically much cooler than surface waters. Cold water is released from Canyon Ferry Dam through the generators all summer long and this cools the river for many miles downstream. As a result, the average July water temperature below Holter Dam (63 degrees) is eight degrees cooler than the average July temperature (71 degrees) of the water flowing into Canyon Ferry. Trout populations begin to become stressed when water temperatures exceed 70 degrees and the cooling effect of Canyon Ferry is another major reason why trout populations are so healthy below Holter Dam.

Do Hauser and Holter Dams control flows in the Missouri River?

No. Hauser and Holter dams are operated by NorthWestern Energy in accordance with requirements of a license issued by the Federal Energy Regulatory Commission (FERC). The FERC license requires Hauser and Holter to be operated as ‘run-of-the-river’ dams that maintain continuous and stable outflows. ‘Run-of-the-river’ dams do not store water, rather, they simply pass whatever water flows into them. Hence, the FERC license requires Hauser and Holter dams to pass whatever flow is released from Canyon Ferry Dam.

Another key provision of the FERC license is a requirement that Hauser and Holter reservoirs can fluctuate no more than one vertical foot, which is much different from the manner in which Canyon Ferry is operated. As mentioned previously, Canyon Ferry Reservoir typically fluctuates around 12 feet annually and because it is a big reservoir, it is able to store a very large amount of water during spring runoff for release later in the year. In contrast, the top one-foot of Hauser and Holter reservoirs contains a relatively small amount of water which is not enough to affect the seasonal flow regime of the Missouri River.

Hauser and Holter dams can have short-term effects on river flow on rare occasions when one or more generators automatically shut down to protect equipment and maintain system reliability. When this happens, river flow is maintained by quickly re-starting the generator(s) or by the automatic opening of spill gates on the dam. These temporary flow interruptions are very uncommon and typically only last a few minutes. Holter Dam is designed and engineered to meet the flow requirements of the FERC operating license and ensure sound river stewardship. The FERC license also allows NorthWestern to request temporary flow increases from Canyon Ferry for up to several days at a time during winter when cold snaps cause major river flow reductions due to ice formation on the 90-mile stretch of river between Holter and Great Falls.

How are Canyon Ferry outflows managed?

Reclamation monitors mountain snowpack very closely and issues monthly water supply reports and operating plans that forecast anticipated Canyon Ferry inflows and outflows. Reclamation coordinates these plans with the U.S. Army Corps of Engineers, NorthWestern Energy, and the Montana Department of Fish, Wildlife and Parks. Reclamation also consults with other stakeholders and periodically adjusts Canyon Ferry outflow throughout the year to ensure that downstream needs for irrigation, flood control, power production, fisheries resources and recreation are met. Canyon Ferry Reservoir level is also carefully

monitored and adjusted to ensure adequate lake levels for irrigation and recreation while ensuring enough space is available to store anticipated spring runoff to reduce the chance of downstream flooding. Copies of the monthly water supply reports and operating plans can be found on Reclamation’s Great Plains Region website: www.usbr.gov/gp or by calling the Montana Area Office of Reclamation at 406-247-7300.

What else does NorthWestern Energy do to enhance and protect the Missouri River below Holter Dam?The federal operating license for NorthWestern’s Missouri River dams contains a number of requirements to protect, mitigate and enhance fish, wildlife and recreation resources. NorthWestern has long-term agreements with state and federal agencies to provide funding and other assistance to meet these goals. In the past decade NorthWestern and its predecessor have contributed over $1 million for cooperative projects to enhance and restore fish and wildlife habitat, monitor fish and wildlife populations, obtain important research and management information, and acquire conservation easements to permanently protect key river bottom habitat downstream from Holter Dam. NorthWestern has also made a number of improvements to the Holter Dam operating system to improve equipment reliability and reduce plant shutdowns which can have short-term effects on river flow.