1927 Guernsey Dam on the North Platte River near Guernsey, WY

(Satellite)

There is an HAER record. But its data file is just captions for the photos. The problem is that none of the photos have been digitized!  

WyoHistory
Guernsey Dam and spillway. U.S. Bureau of Reclamation.
"Guernsey Dam is a 135-foot-high, diaphragm-type embankment of sluiced clay, sand, and gravel....200 laborers were at the site at the mouth of the small canyon, using steam shovels to gather broken rock for the dam’s embankment. Trains hauled the material to the dam site and dumped it from trestles on each side of the embankment. Completed on July 13, 1927, Guernsey Dam stores water and also controls flow from other dams on the river."
[I wish I better understood where the sluiced material was placed in the dam. I've read that sluiced dams are now considered dangerous. Most of the article explains that Guernsey State Park was a pioneer development of recreation sites.]

The two-generator powerhouse with its surge tank is in the left background, its gatehouse is in the foreground, and I think the two gates in the right background control the input to the powerhouse.

Street View, May 2012

In the satellite image, there is no water flowing in the spillway. But in the street view below we can see they are releasing water in that spillway. That release would explain why we see a gap at the top of the counterweight stack in the above street view.

Street View, May 2012

US Bureau of Reclamation
Spillway Capacity At Elevation: 52,000 cfs at top of active conservation pool of 4420'.
Top of inactive conservation pool: 4370'
Crest Elevation: 4430'
Crest Length: 560'
Auxiliary Spillway: No [more on this later]
Hydraulic Height (Normal Operating Depth at Dam): 96'
Net Generation: 12,108,622 kWh.

Does net generation mean the total during its lifetime of 94 years? I think so because the installed capacity is low.

USBR-power
It was built with 4.8kw. During 1992-94 the generator windings were replaced to provide an installed capacity of 6.4kw. "Guernsey Power Plant is operated on a seasonal basis during the release of irrigation flows to satisfy downstream demands on the North Platte River in Wyoming and Nebraska."
Turbine Type: Francis   Rated head: 70'   Power Factor [Percentage of the year that it runs?] 26.1%
  "An emergency contract was awarded and completed in FY 2003 for stabilization of rock on the hillside above the power plant and transformer deck to prevent a potentially catastrophic rockslide that might have damaged the power plant and transformers and resulted in an oil spill into the North Platte River." 

USBR-production

USBR via NPS

USBR via NPS
The dam and powerhouse under construction. Bureau of Reclamation

USBR-data
* Reservoir is considered "full" when pool elevation is at top of active conservation pool. Percentage is based on total reservoir volume below that level.

This is the second dam I remember seeing with a "trench spillway." I saw the first, Sanford Dam in Texas, just a little over a month ago. I think that dam has the deep spillway because the watershed is in a desert. But the Rocky Mountains are not a desert. I also noticed that they do not specify an elevation for the top of a flood pool. And the conservation pool is within 10' of the crest. The above "triangle diagram" reinforces that there is no flood pool. The following diagram for the Keyhole Reservoir, WY, shows what the "triangle diagram" looks like for a dam that does have a flood pool.

USBR-keyhole

That would explain why Guernsey has a deep spillway with a big sluice gate at the opening. It needs to be able to release a maximum rain event rather than retain some of the water for flood control. And I have concluded that the structure in the satellite view below is the input to the hydropower plant. That means the dam can generate power only when the reservoir is full, which makes sense since it doesn't do flood control.

Satellite

You can tell this is a wilderness area because Global Earth doesn't have a lot of images of the dam. This one has the maximum flow in the spillway. It appears that they are releasing water even though the reservoir is not full. This confused me until I read that the outflow supplies water for irrigation. Obviously, irrigation would be higher power than a little power generation.

Global Earth, Aug 2009