Layman’s Guide to South Fork Landslides

With the number of slides that occurred from the rain on September 12 we thought it would be useful to provide a primer on the six slides that have been documented.  Many of the photos came from different news sources and government agencies.  Not all photos were specifically located so a field recon on Monday aided in getting a sense of what happened last week.

A couple of things first: the road is closed from Highway 20 to the Anderson Ranch Dam and down along the South Fork Boise.  The road from Highway 20 to Cow Creek bridge is open, and continues to be open downstream to Prairie and beyond.  Upstream of Cow Creek bridge the road is closed (as of September 17).  Second, the water is murky in the South Fork not only because of the mud flows and land slides that reached the river, but also because of the turbidity in the water coming out of Anderson Ranch Dam.  There is a lot of silt that is in suspension in the reservoir.

Each slide is presented in order starting upstream and working in a downstream direction.

1. Approximately one mile downstream of Anderson Ranch Dam you reach the boat ramp (a.k.a. Tailwaters) and you see the first landslide:

Landslide downstream of boat ramp. Photo from KTVB

Mud and debris plugged the side channel on river right and created a rapid which currently has some snags.  Navigation could be a problem until the trees are washed down or removed.  The side channel will not return unless higher flows scour the channel.  The slide also put enough material in the river to raise the water elevation and create a slow water area, probably a couple of hundred yards in length, upstream of the slide.  It is similar to what anglers and boaters have experienced upstream Rough Creek since the landslide in January 1996.

2. Downstream a few hundred feet there is a separate slide that appears to have delivered less volume to the river, but nonetheless put a lot of mud and debris on the road.

Second mudslide is just downstream of the first one. Photo by John Wirt, US Geological Survey

Some anglers refer to this general area as the Owl Rock Hole.  The rock outcrop partially obscures the river when the photos are from the road, high above the river.

3.  Just east of Reclamation Village a slide covered some 700 feet of road with mud.

Mud slide east of Reclamation Village. Photo from Boise National Forest

Approximately 3,000 cubic yards of material will have to be removed from the road.  The photo indicates much more than that crossed the road, much of which entered the South Fork.

The photo below is another angle and shows the Anderson bridge upstream of the slide as well as how far across the river the slide reached.

Mud slide at Reclamation Village puts the squeeze on the River

Some anglers will remember on the south side of the river the large rock outcrop with a deep hole below it.  Wherein lurk some large trout.  The early report is the deep pool appears to be more of a run than a pool.  And the photo does appear to show a some mud across the river at the subsurface.

4. From Reclamation Village it is approximately 4.5 miles downstream to the confluence with Rough Creek.  There may be slides between these two points but no identifying photographs have been produced.  And the road is closed in this reach so any field recon effort is not possible.

Rough Creek and South Fork Boise River road

At Rough Creek a major slide pushed across the road, covered much of the slide area of 1996 and dropped into the river.  It appears the 2013 edition of the Rough Creek slide is different than the 1996 event, at least from the photographs and inspection of other slides.  The new one resulted from the rain after fire removed nearly all vegetation in the catchment.  There is a lot of sand and silts in the mud, and some coarse material typical of the Idaho batholith.  The slide in January 1996 was very different because it had more evidence of a mass failure dominated by large sized rubble and boulders moving into the river.

View looking downstream with Rough Creek slide coming from the right.

The extent of the mud flow versus the water is evident in the above photo.  Much of the profile line of the mud flow approximates the slide from 1996.  It appears that the amount of debris on this slide was relatively less than at Reclamation Village. The slide area and rapid is a confined river channel thanks to the rubble that remains from 1996 and the water velocities are relatively higher in this area.  Even at the 600 cfs flows as in the photo above there is already a demarcation between the river flow and the mud flow.

5.  The big debris flow occurred at Granite Creek.  We already provided some background on this blowout.   If anything, the scale of the event was underestimated.

South Fork Boise River and Granite Creek debris flow. Courtesy USDA Forest Service.

The photo at left shows the debris fan well into the river and then it appears to taper off to river right along a 500 foot length of channel.  Secondly, there is significant change to the backwater created by the mudflow as the river banks are flooded and the river much wider.  The streamside willows are now nearly in the middle of the river!

What the photo does not show is the extent of the debris that washed downstream around the bend in the river.  The Rock Garden is completely covered. The Pine Tree Hole is no more.  The hole filled with mud and debris and now has a fast moving riffle cutting through the deposit.

This is where the Rock Garden used to transition into the Pine Tree Hole. Note the large tree on the new gravel bar.

In the photograph above and island could form from the gravel bar if high flows do not move the tree.  Vegetation can establish and stabilize the deposit. 

For nearly one-half mile – from Granite Creek around the bend - the river takes on a series of S curves, as illustrated in the photo below.

View upstream at what once was the Rock Garden

This new channel is cutting through the mud flow and will continue to change the form of the stream as there is no stream side vegetation to stabilize the banks.  And it appears much of the material may be appropriate sized gravel suitable for spawning, assuming the smaller sands and silts can be washed away.  Concerns about a lack of spawning habitat in the main river may have been addressed.

Around the bend, downstream of what used to be the Pine Tree Hole

One takeaway from seeing the extent of the mud slide from Granite Creek is that anglers will need to be aware of safety hazards associated with the relatively loose material deposited along the stream bottom as well as the significant amount of trees and associated debris.

One gravel bar made up of material from Granite Creek is nearly one-half mile downstream.  The Granite Creek bomb slide event appears to have won the prize for affecting the most area along the river.

6. The final mud flow and slide is at Pierce Creek, and there is some good news here.  When the Forest Service initially circulated aerial photos of the mud slides across the road and into the river Pierce Creek was not included.  This is because the new bridge at Pierce Creek was designed and built to pass more water under it.  And it worked.  And because it worked there was little in the photograph to see, at least across the road.

The mud in Pierce Creek went under the bridge.

It appears the slide from Pierce Creek is on the scale with Rough Creek, neither being as large as Granite Creek.  But still significant nonetheless.

The bridge was constructed in 2011 as a cooperative project with a number conservation groups and agencies to reconnect Pierce Creek so trout from the South Fork Boise River could access it.  A four foot dimeter culvert was replaced with a bridge with a much larger opening that in this instance was able to pass the mud flows and debris into the river.

Much of the grade control structure of Pierce Creek downstream of the bridge was blown out.

One effect from the magnitude and character of the mud flow was its impact on Pierce Creek.  Much of the rock placed in the simulated channel was washed out and this will need to be replaced so that fish passage can be restored.  The photo above shows the rock on one bank was either scoured away or covered with mud.  On the other side of Pierce Creek some of the vegetation appears much less affected.  Some very large foundation rocks in the stream bottom held in place as the waterfall is evidence of the loss of a more gradual step pool channel.

So that’s it for where the slides occurred and what they look like.  It’s way to early to know what this all means for the South Fork Boise River.  Much clean up and repair is occurring.  Rehabilitation of the burned areas will commence this fall and mulching and seeding activities will help the land recover and stabilize so these type of events will be less likey to happen in the future.

 

 

Video and pics on several South Fork landslides

The Granite Creek Bomb

Among the debris flows into the South Fork Boise River on the evening of September 12, Granite Creek is worth a look.  For one reason, we said it was a ticking time bomb here and here.  Secondly, the debris flow appears significant in its scale and extent.

Map highlighting the Granite Creek Watershed

First, the map above shows the watershed is about halfway between the Cow Creek bridge and the Danskin Bridge.  Granite Creek is a tiny little thing with average flows in the winter months of a couple of cubic feet per second.  In the spring there could be 10 to 20 cfs in the stream, based on a statistical report.  All of this fits in a culvert under the South Fork road with a pipe of 36 inches in diameter.

We know from the 1978 study of the South Fork and tributaries that Granite Creek did have some documented spawning in it.  In more recent years the culvert is considered an impediment to aquatic organism passage.  Attempts to assess the culvert’s outflow were frustrated by too thick vegetation and steep slopes off the South Fork road.

The Google Earth view of Granite Creek watershed.

There is (or was) some forested sections in the Granite Creek watershed, primarily on the north facing slopes.  It is approximately 3.5 square miles in size.

The photo from the air on Friday the 13th is dramatic:

South Fork Boise River and Granite Creek debris flow. Courtesy USDA Forest Service.

First, the debris flow overtopped the South Fork road as the little culvert was either plugged or overwhelmed.  The debris fan extends well downstream as indicated by the tan or lighter-colored area in the South Fork Boise River, when compared to the chocolate color of the river upstream and on river left as it routes around the debris flow.

Another shocker is from the photo below using the Google Earth image generally in alignment with the above photo taken on Friday:

Google Earth angled image of Granite Creek confluence with South Fork Boise River

Both photos show a river flowing at 600 cfs as the Google Earth flyover was August 29, 2011.  The debris flow as apparently covered the Rock Garden section of the South Fork, burying that stretch of pocket water just as it enters the Pine Tree Hole.

More amazing is the amount of water backed up with the mud and debris deposited in the river.  The water is backed up and at a much higher elevation, so much at the willows and other stream side bushes are inundated, surrounded by muddy waters.  This is most apparent along the left bank.  At the bottom of the slope the group of trees appears in water too!  The water elevation appears to be 2-3 feet higher than normal.

This condition will not last as the river will continue to cut a channel through the debris and the smaller size material (silt and sand) will be carried downstream.  That Pine Tree Hole is going to be full of material for some time.

How fast the river cuts a channel and clears out the smaller material is going to be affected by how much flow is in the river.  The winter time flows of 300 cfs commence on September 16, so what ever energy the river has to move the material downstream will be greatly reduced.

A natural follow-on question is the effect of this and the other debris flows on the South Fork Boise River fishery.  This is a question much more difficult to determine at this time.  The extent of the numerous locations where flows entered the river and apparently blocks flows on Thursday the 12th must be considered in a cumulative fashion.  A full accounting of the flows and estimates of each disturbance event would be a good first step.

A further consideration is that while fish populations can be affected by these type of disturbances and recover over time, the cumulative effect of several debris flows into a river that has a controlled flow release regime could mean the recovery of the river will be dependent on how the water releases are managed in the spring and summer months.  For example, could artificial spring freshets be managed in order to help move sediment through the river system?  This is a topic that conservation and angler groups will need to work with the government agencies and other affected water users.

 

RAIN CHANGES EVERYTHING: Mudflows in South Fork Canyon blocks road, inundates river, closure continues

So much for that plan to open a portion of the South Fork Boise River.

The email message from the Boise National Forest’s Mountain Home Ranger District:

Guys, due to the mudslides and ongoing instability of the area, the SF Boise River will NOT open as discussed at the TU meeting Wednesday. The area will remain closed until further notice. As of now all access roads to the SF are closed. Again the area is closed to everyone except Federal, State or County personnel in performance of official duty.  

The rain gage at Anderson Ranch Dam recorded more than 0.6 inches of precipitation on Thursday evening September 12.  This piece of data portended the mudslides, gullywashers and mass failures that have inundated the South Fork Boise road and reached the river in many locations.  For example the photo below from KTVB.com shows a big mudslide.

Mud slide across South Fork Boise road just east of Reclamation Village. Photo from KTVB.com website

News coverage with some photos can be found at the Idaho Statesman and at KTVB websites.  There is also a very good overview news release from the Boise National Forest here.

 

 

Fire Effects Discussed; Partial Opening of River Imminent; Stranding Study Slated September Sixteenth

Good attendance for community meeting on South Fork Boise River

Approximately 75 people made it to the MK Nature Center Wednesday Night to get the low down on the effects of the wildfire that affected the South Fork Boise River.  The audience got a dose of information where things stand with the South Fork.

Mountain Home District Ranger Stephaney Church said she is working to open the section of the South Fork of the Boise River from the Cow Creek bridge downstream.  Perhaps as soon as this weekend.

Church said the section of the river was less affected than from Anderson Ranch Dam to Cow Creek.  This website back on August 21 showed the extent of the fire in the lower section of the river.  There were fewer trees burned along the river and therefore less work to be done to cut down the trees that could be a safety hazard.  Further upstream its a different story and it will take much longer to assess the safety hazards and do something about them.

One questioner missed badly when stating that everyone attended the meeting just because they want to know when the river will be open again to fish.  Visits to this website and discussions among the fishing and conservation groups show otherwise, as there is significant interest in the long-term future for the South Fork fishery and for the river ecosystem.

Dick Frencer of Boise Valley Fly Fishers poses a question to Stephaney Church

Agency staff with the Forest Service, BLM and Idaho Fish and Game described the effects of the fire and the field assessments done so far so a game plan can be developed to address the most immediate needs for emergency rehabilitation.  While the meeting discussion was at a more general level, some specifics have emerged.  For example, the Elk Complex fire north of the river experienced much severe burning and the effects on soils in some locations appears fairly dramatic.  We are keeping an eye on Pierce Creek, Granite Creek and Rough Creek.  All three drain into the South Fork of the Boise from the north side.  Granite Creek in particular could be a ticking time bomb with a small culvert and a watershed that appears to have seen severe fire effects.  Pierce Creek of course no longer has a small culvert but rather a new bridge, so if there is a gully washer one hopes the flows and debris should be able to pass under the bridge.

Meanwhile the stranding study will proceed with government agency staff biologists on Monday, September 16 when the South Fork Boise River flows will drop to 300 cfs.  Idaho Fish and Game and the Bureau of Reclamation will have staff on site to ascertain the amount of stranding when the river flows are cut.  This information will help inform future management changes with river flows.