flows

The Late August Flow Surge

The Bureau of Reclamation announced in an August 13 news release that flows on the South Fork Boise River will rise to 2,400 cfs for several days to transport sediment in the river and help improve fish habitat.

This action comes after several months planning a response to the August 2013 wildfire which burned much of the SFB watershed, followed by heavy rains September 12, 2013 that caused several debris flows that delivered sediment and debris into the South Fork Boise River.  Additional storms this summer have added more material and debris to several locations along the South Fork.

Granite Creek blowout upstream of the South Fork Boise River Road.

Granite Creek blowout upstream of the South Fork Boise River Road.

A South Fork Boise Recovery Team; including US Forest Service, Trout Unlimited, University of Idaho, Idaho Department of Fish and Game, and U.S. Bureau of Reclamation met and developed flow recommendations and a rationale.  Here are some highlights of their information:

Fire and the resulting sediment and debris slides are a natural and regular occurrence in southern Idaho and the Idaho Batholith. In natural systems, fires impact fish populations positively and negatively. In managed systems, impacts may be more or less severe depending on circumstances.

Changes in sediment regimes may be one of the largest changes to stream networks post fire. A tremendous amount of fine sediment was added to the SFBR during these events. Fine sediments affected the quality and quantity of overwintering habitat by filling in side channels and interstitial spaces along the shoreline, these habitat features are critical for the survival of bull trout, rainbow trout, and their prey bases. Reduced quality and quantity of overwintering habitat and spawning habitat for prey fishes affect bull trout. Fisheries surveys in the fall of 2013 and spring of 2014 show reduced numbers of age-0 rainbow trout that will support the future rainbow trout fishery and serve as prey fish for bull trout. In a natural system, these sediment inputs are sorted and transported under naturally occurring high flows during spring snowmelt. In regulated systems, such as the SFBR, high flow events are attenuated to mitigate flood risk and to store water for irrigation (and other uses). The regulated flows result in lower peak flows especially in poor water years or years with low carryover storage like the winter of 2013/2014. During these conditions, less fine sediment is transported leading to longer-term detrimental impacts to bull trout, their habitats, and other aquatic biota.

The multi-agency group held meetings November14, 2013, March 5, May 29, and July 18, 2014.  In addition to developing a list of data needs to assess potential responses to mitigate the effects of the fires and resulting debris flows, and discussing results from sediment modeling and recent fish surveys, the team recommended flow shaping to:

  1. Provide as much stability as possible during the rainbow trout spawning period;
  2. Provide stability for incubating rainbow trout eggs; and
  3. Increase reservoir storage, in anticipation that increased storage would enable a flushing pulse that could be used to mobilize fine sediment during the 2014 spring runoff period.

Storage in the Boise River system precluded a flushing event in the late spring as there was little room to store the flushed water in the lower system (Arrowrock and Luck Peak reservoirs). Thus the team “believes that a pulsing flow is an important step to hasten recovery of important fish populations and habitats found within the SFBR. As such, the team reconvened after learning that a spring flush was not possible to update the sediment model and discuss options for a flushing pulse later during the 2014 water year.”

South Fork Boise River flows cut a channel through the sediment.

Summer 2014 – South Fork Boise River flows cut a channel through the sediment.

The University of Idaho’s Ecohydraulics group developed a 1 dimensional (1D) coupled hydraulic and sediment transport model of the river from  Anderson Ranch Dam Gage to Neal Bridge to simulate sediment transport through the system.

The volume of recently deposited sediment (debris fan) was estimated by field measurements.  Five debris fans were identified within 10 miles downstream of Anderson Ranch Dam. The 1D sediment model predicts that size of sediment that will move under certain flow volumes given the channel cross-sectional shape and slope. Sediment particles less than the size used by spawning fishes were considered in the model.

Existing LiDAR DEM data was used to extract cross-sectional depth data to more accurately identify just how deep some of the recently deposited sediment fans are, and to identify where redistribution of sediment would occur.  Three

Movement of sediment depends on speed of water and size of sediment particles.

Movement of sediment depends on speed of water and size of sediment particles.

different discharge magnitude and duration scenarios (recommended by the team) were used to investigate total volume of sediment transport from debris fans, upper canyon and canyon sections. Discharge scenarios included a pulse of 2000 cfs for 8 days; 2400 cfs for 8 days; and 3000 cfs for three months.

Sediment transport simulations based on the 1D model showed that the 2014 irrigation flows of 1600 cfs move some sediment but do not erode the debris fans completely. Part of the mobilized sediment is deposited in lower velocity reaches like pools and the remaining sediment remains in transport.

Key findings from latest modeling:

  • A flow pulse of 2000 cfs for 8 days does not visibly change the sediment transport.
  • A flow pulse of 2400 cfs or greater for 8 days causes a detectable change in the deposition of sediment while improving the quality of both overwintering and spawning habitat.
  • Flows in excess of 3000 cfs, sustained for as long as three months will mobilize fine sediments but will not flush them out of the river corridor into Arrowrock Reservoir.

Analysis of potential depositional areas shows that overwintering habitat and spawning reaches should not experience large deposits of fine sediments, improving the quality of both habitat types.

We recommend a flow pulse of at least 2400cfs for a minimum of eight days based on the new results from U of I’s sediment transport model.

The Bureau of Reclamation is required to protect critical habitat for bull trout habitat, thus the water storage levels in both Anderson Ranch and Arrowrock reservoirs require attention during the same time period as the recommended pulsing flows. Neither requirement will be jeopardized as a result of the recommended flow pulse. Furthermore, water used for the flushing pulse (approximately 8,000 acre feet), if stored in Arrowrock Reservoir through November 2014, would provide an additional benefit to migratory bull trout returning to Arrowrock Reservoir.

The flow increase is scheduled to begin Monday the 18th.

 

 

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The 2013 Weak Water Year

We grabbed a few charts at the end of Water Year 2013 to put some perspective on the water situation.

Flows from Anderson Ranch Dam:

Flow From Anderson Ranch Dam where blue line = 2013, green line = 2012, red line = long-term average

The summertime flows were 1,600 cfs instead of 1,800 cfs in 2012.  The 1,600 cfs flows extended well into August to provide irrigation water to users in the Treasure Valley.  The rest of the system was draining so more water was moved from Anderson Ranch pool.  And you can see what happened in the reservoir here:

Water stored in Anderson Ranch Dam blue line is 2013 and green line 2012 and long term average is the red line

Anderson Ranch reservoir started the water year (October 2012) very near the average for water in the pool.  The low snowpack in the winter meant less runoff into the pool in the spring and early summer.  The peak amount of water in May was nowhere near the average, let alone filling Anderson Ranch.  The the 1,600 cfs flows pulled the contents to an amount well below average and at a high rate until mid August.  The water year ends with 100,000 acre feet in the pool.

The reservoir drained very quickly because it was a year where much less water flowed into Anderson Ranch Reservoir as seen in the chart for the water gage at Featherville:

Water flows on South Fork at Featherville gage with 2013 in blue, 2012 in green and average in red

So the blue line in the chart above shows just how below average the flows were upstream of Anderson Ranch pool.  By the way, the spike in river flows in late April 2012 on the green line is a same time when a record flow was observed in the Middle Fork Boise River at Twin Springs gage.  Some readers may recall the late April heavy rains that  melted much of the lower to mid elevation snow into the South Fork Boise River.

One indication of the water year ending in a whimper is what happened with the flow gage on the South Fork Boise a mile or so downstream of Anderson Ranch Dam:

River flow data in late September 2013 was affected by the landslides into the South Fork Boise

The slides on September 12, 2013 knocked out the water gages for a few days.  They are back running now, but the chart above from September 24 shows only a brief period when it was working that day and no data was charted for previous days.  This situation is understandable given that much debris tumbled into the South Fork and it may have directly affected the functioning of the gage or perhaps changed the profile of the channel in the area of the gage making a clean reading of water flows a challenge and thus a need to recalibrate the equipment.  The automated system for this gage is working as shown on the USGS gage here.

Looking forward, there have been some inquiries and informal discussion whether it would be possible to shape flows from Anderson Ranch Dam in 2014 to deal with the debris and sediment in the South Fork Boise River to help transport the sediment and allow the river to carve a channel through the massive deposits in the river.  For any of this to happen it will require water in Anderson Ranch pool that is not yet there.  A winter with above average snow could be very important to developing some alternatives to shape the flow releases, their timing and magnitude, and it will have to happen without adverse effects on downstream water users.  But readers may be interested in knowing it is a topic of conversation, or at least was prior to the government shutdown.

 

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Flows drop after a long pull on Anderson Ranch Water

 

South Fork Boise River flows downstream of Anderson Ranch Dam

The Bureau of Reclamation began turning down the dial on the boating flows after the end of the Labor Day weekend.

By the end of Tuesday September 4th flows dropped from 1,850 cfs to 900 cfs.  On Wednesday the 5th the flows were at 600 cfs.

Summer boating flows lasted longer and were slightly higher than the typical 1,600 cfs.  In fact, the 1,800 cfs flow seems to be a “new normal.”

So why did flows stay higher through Labor Day?  Continue Reading…

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Flow management and stranding juvenile trout: what we know and don’t know, and what it means

If you are interested in helping study the effects of the South Fork Boise River flow changes on the wild trout fishery in September, read more below and contact us with your availability.

For several years the South Fork Boise River wild trout fishery has been getting significant attention from a number of sportsmen, state and Federal resource agencies.  You reading this website story is just one example.  River flow changes and effects on the aquatic system has gained attention with angler concerns about potential adverse effects on juvenile rainbow trout and macroinvertebrates (aquatic insects that provide a food base for trout), when Anderson Ranch Dam flows are routinely ramped down to 600 cfs and 300 cfs as irrigation storage releases are ended in September.

Whitefish Ed in his formal fishing dress on the banks of the Henry's Fork (photo from westfly message board)

Back in November 2009 interested anglers and agency staff met at the Boise Public Library to hear about a genetic study of the South Fork Boise River fishery.  During a panel discussion several topics came up, including flow management.

Could the Bureau of Reclamation do something to change their flow management?  There was no answer to that question, but the topic did stick with one local angler who goes by the handle “Whitefish Ed” on the westfly.com message board.

Ed initiated his own study and looked at the SFB on September 15, 2011, a few hours after the flow dropped from 600 to 300 cfs, and observed many stranded wild juvenile trout along the river’s shoreline in three locations.  You can find Ed’s story at this westfly.com message thread which is worth the read.  Ed’s writing style is as colorful as that shirt he is wearing in the photo

After some meeting this spring with state and Federal agency biologists a plan has Continue Reading…

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South Fork at 6,000 cfs: A Top Ten Rating

On Friday April 27, 2012, releases from Anderson Ranch Dam hit 6,000 cfs.  This is a flow quantity rarely seen on the South Fork Boise River.  Based on records dating back to 1943 (seven years before the dam’s completion) here’s the top years where peak flows got nearly to, or exceeded, 6,000 cfs.

Since 1943 here are the years where peak flows were 6,000 cfs or better

Continue Reading…

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