Bridge Rebuilt

The bridge abutments on the Pierce Creek bridge have been replaced.  The project is a cooperative venture of the Mountain Home Highway District and the Boise National Forest. The abutments are treated laminated wood beams and posts, and driven several feet into the ground next to the stream.

The bridge over Pierce Creek, a spawning tributary to the South Fork of the Boise River

The project was completed in fall 2018 and secures the bridge for the long term.

A mud crust over the plate on the bridge. Mud deposited from the numerous debris flow episodes 2013-2014.

The bridge was placed on its original sill in November 2011.  The Elk Complex Wildfire of 2013 led to massive debris flows in Pierce Creek and ultimately would have compromised the foundation for the bridge. So the decision was made to life the bridge and replace the substructure and put it back in place, which happened in fall 2018.

Future high flows and flooding will easily pass under the bridge. Pierce Creek fish passage to its headwaters will continue.

Bridge repairs cause road closure along SF Boise River

News Release   

Boise National Forest

Contact: Mike Williamson

Phone: (208) 373-4105


 Bridge repairs cause road closure along SF Boise River

Boise, Idaho, Aug. 27, 2018 — The Mountain Home Highway District will enact a closure on a portion of Prairie Road (Forest Road 121), located about 7 miles south of Prairie near the Danskin boat ramp, beginning Sept. 5 for around 14 days while repairs are made to the Pierce Creek Bridge footings.

This closure will affect boaters on the South Fork Boise River who typically end their float at the Danskin boat ramp, as it will prevent road access to the ramp from upstream. The ramp itself will remain open, but only accessible if driving in from the north.

“Closing this portion of the road is expected to impact recreationalists, local residents and businesses,” said Acting Mountain Home District Ranger Holly Hampton. “We ask that travelers in the area reconsider their routes.”

Since the 2012 Trinity Ridge Fire, there has been a significant amount of debris flow on Pierce Creek that has impacted the bridge footings and making repairs necessary. This work is being done with funding from the Forest Service and MHHD.

This portion of the road is managed and maintained by MHHD.

For more information contact the Mountain Home Highway District, 208-587-3211.

(see attached map)


Sediment Movement shown in Photographs

Here are some photographs showing the movement of sediment from the Granite Creek area where it enters the South Fork Boise River just upstream of the “Pine Tree Hole” a mile or so downstream of Cow Creek bridge.

The debris or mud flows occurred on September 12, 2013.  The first set of photos was taken Sept. 16, 2013.  A comparison photo for the same area was taken Sept. 22, 2017.  River flows on both days was 300 cfs.


Enough of the burned tree trunk remains to line up the photo point.  Smaller sediment is scoured and gone, as is the wood.


That big tree bole in the upper photo is long gone.  When flows are at 7,000+ cfs it floats the logs downstream.


Upper photo just three days after the debris flows and a sinuous channel was routed through the sediment.  Four years later the smaller particles are gone and channel is widening, exposing the larger rock that was deposited from the Granite Creek blowout.


The big pile of trees on the large rocks are now washed away.  Moreover the large boulders are now re exposed after the river flows scoured the sediment.


Looking downstream the deeper hole appears to have reformed.  In the distance the hillside appears barren of sage brush and bitter brush.


Moving downstream a couple hundred years and then during the photo looking upstream the debris has been moved out of this section.

A final photo set, looking upstream, at the head of the pocket water above the pine tree hole.


Summer Impression of Water Flows

A summer drop in on the South Fork Boise River.  A quick stop or two to observe the status of the river after the very high flows in the spring and early summer and it appears much of the sediment has been moved out.  Check out this photo of the pocket water emerging again before it dumps into the Pine Tree Hole:

Looking downstream towards the Pine Tree Hole. Early August 2017

More interesting is if you compare this photo of a broader area to that take after the debris flows into the South Fork in September 2013.  Here is a photo that is trained on the conical tree on the other side of the river:

This is where the Rock Garden used to transition into the Pine Tree Hole. Photo taken September 2013.

Two things are different, three really, from these photos.  First, the sand bar evident in 2013 is less so in 2017 so some of it has washed away.  Second, the wood piled up on the bar and on the right bank is pretty much washed away into the South Fork canyon and ultimately Arrowrock Reservoir.  Third, yes there is a third thing, the flows in summer 2017 are in the range of 1,800 cfs and the flows September 16, 2013 was the first day at 300 cfs.  So this fall when the river is at 300 cfs we will go back and get some photo point updates for a more straightforward comparison.


Sunday Trip

Sunday we took the Anderson Ranch to Prairie to Black Creeks road trip. Took a couple of pictures.

Was surprised by water being released using the spill way.

Water level at the main boat ramp.

Met one of the locals.

Decided not to go camping at this site.