What causes flow fluctuations at Ashton Dam? HFF solves long-standing mystery.

Graphs of river stage at five locations on Henry's Fork.
Graph of river stage above and below Ashton Reservoir.
Graphs of river stage above and below Ashton Reservoir; close-up.

For many years, outfitters and guides, anglers, and irrigation managers have inquired about the causes of daily fluctuations in streamflow in the Henry’s Fork downstream of Ashton Dam. These fluctuations are large enough to be noticed by anglers while they are out on the river and large enough to cause canal diversion rates to vary throughout the day. Although fluctuations occur year-round, they are highest in amplitude and have the greatest effect on fishing, river ecology, and irrigation operations during the late summer and early fall.

The most common belief among those who observe streamflow downstream of Ashton Dam is that the fluctuations are deliberately caused by operation of the dam and reservoir. Ashton Reservoir is not an irrigation storage reservoir; it exists solely to provide hydraulic head (vertical drop) for the hydroelectric power plant at the dam. The power plant’s Federal Energy Regulatory Commission license stipulates that the plant must operate as a “run-of-river” facility, which means that outflow must equal inflow. So, either the power plant is not being operated as a “run-of-river” facility or the flow fluctuations originate upstream of Ashton Reservoir and are not created by operation of the power plant.

However, there is no stream gage station located immediately above Ashton Reservoir, so we have no record of inflow to compare with the outflow, which is recorded at the USGS Ashton gage station. As most of you know, this station is located at the Ora Bridge boat launch, a little less than one mile downstream of Ashton Dam. In fact, there are no stream gages between Island Park Dam and Ashton Dam, a distance of over 48 river miles. Without stream gaging in that reach, it is impossible to answer the question of what causes fluctuations in river downstream of Ashton Reservoir.

During August of 2013, when daily fluctuations in flow were very large, my wife Sheryl and I measured stream depth upstream and downstream of the reservoir every few hours over a 24-hour period. Yes, we really did get up in the middle of the night to do this. Our measurements showed the same pattern of daily fluctuation above the reservoir as below the reservoir. Of course, a few measurements made with a depth rod wouldn’t have much statistical power, so HFF decided to install a $15,000 recording device upstream of Ashton Reservoir to provide the data needed to answer the question.

Well, sort of. The device really did cost $15,000, but it measures a lot more than just depth. In fact, we bought and installed four of these instruments, which are called “sondes,” as part of our new water-quality monitoring network. In 2014, we installed sondes in the Henry’s Fork at Flatrock, Island Park Dam, Pinehaven, and the old Marysville bridge, which is about 1.6 miles upstream of Ashton Reservoir. HFF’s water-quality monitoring program was reviewed and endorsed by the Henry’s Fork Watershed Council in April of 2014.

To address the Ashton flow issue, I analyzed river stage, as recorded every 15 minutes at USGS gage stations at Island Park Dam, Ashton Dam, and St. Anthony and at our sondes at Pinehaven and Marysville. Actually, the instruments at these stations measure pressure. When atmospheric pressure is subtracted from the total pressure, the remaining value is due only to the weight of water above the instrument. Using the known density of water, the pressure is then converted to water depth, which is then related to what hydrologists call river “stage.” Stage is really just a “yardstick” that measures height of the water surface relative to the bottom of the stick. Ultimately, stage is converted to streamflow, using an empirically derived statistical relationship between flow and stage.

For the purposes of this analysis, I transformed the stage measurements onto a common scale, so that changes in stage were proportional to changes streamflow, thereby allowing “apples to apples” comparison across the five gages. I performed a detailed comparison of stage at the USGS Ashton gage (below Ashton Reservoir) with that at our Marysville site (above Ashton Reservoir) over the period July 8, 2014 to November 12, 2014. You may recall that the river and reservoir froze on November 11, and we chipped our sondes out of the ice on the 13th to download data and store them for the winter.

Daily cycles originate in Harriman reach

Focusing first on the week of September 5 through September 11, when daily fluctuations were at their 2014 peak, it was immediately apparent that the fluctuations originating in the Harriman State Park reach of the river and travel downstream from there. This is clearly shown in the first figure above. Mouse over the figure to read the caption, and click to enlarge the image to full-screen mode. These stage data, along with additional data we have collected over the past two years, indicate that the daily fluctuation in flow is caused by daily cycles of respiration and photosynthesis in rooted aquatic plants (“macrophytes”). Throughout the day, plant tissues expand, increasing their volume and temporarily storing some of the river’s flow. Shortly after sunset, the plant tissues contract, and the stored water is released.

You can see in the graph that at Pinehaven, immediately downstream of the largest concentration of macrophytes in the river, streamflow is lowest right around sunset, when photosynthesis ends, and highest just after midnight. The shape of the cycles is retained all the way to St. Anthony, as they travel down the river. Notice that travel time between Pinehaven and Marysville (27.2 river miles) is about 8 hours, so by the time the cycle gets to Marysville, streamflow is lowest at around 4 a.m. and highest just after 9 a.m. Another hour-and-a-half (7.7 river miles) later, the cycle gets to the USGS Ashton gage, where the minimum occurs at around 5:30 a.m. and the maximum at 10:30 a.m. A little less than two hours are required to travel to St. Anthony (another 11.8 river miles), where the minimum occurs a little after 7:00 a.m. and the maximum occurs just after noon.

Effect of Ashton Reservoir

In the second figure, I compare river stage above and below Ashton Reservoir. Again, this is an “apples to apples” comparison; stage has been adjusted for travel time and for differences in channel configuration so that the resulting comparison is a direct comparison of inflow and outflow. It is immediately obvious from this figure that outflow equals inflow; the vast majority of variation in flow below Ashton Dam simply reflects variation in flow above Ashton Reservoir. Streamflow at both locations reflects the following features:

  • Flow adjustments at Island Park Dam made on July 8, August 8, August 22, September 3, October 1, and October 3.
  • Rainstorms on July 21, August 3, August 20, August 29-30, September 27-30, and November 1.
  • Daily macrophyte-driven cycles beginning around August 1, peaking in early September, and ending in mid-October.
  • Macrophyte-driven cycles absent or greatly decreased in amplitude during rain events.
  • Greatly decreased streamflow during freeze-up on November 11.

The third figure shows a close-up of the comparison during early September, when macrophyte cycles were at their peak. The cycles have an amplitude of around 2 inches (2 inches below average at the minimum and 2 inches above at the maximum). Note that the daily cycle was very muted on September 11, which was a very cold, cloudy day, when little photosynthesis occurred. This figure also shows a close-up of the week during freeze-up. Note the dramatic drop in flow on November 11 and 12 as the river froze. Tying up that much water in ice over a short time period definitely reduces river flow.

This third figure also illustrates that even though outflow primarily reflects inflow, there are additional fluctuations present below Ashton Dam that are not present above. These differences typically result in stage differences of around one-half inch; in fact, over the July 8 to November 12 period, these differences averaged 0.48 inches, which equates to about 50-100 cfs of flow at Ashton the Ashton gage under late-summer conditions. This is about 4-10% of the river’s flow.

Quantitative breakdown of flow fluctuations  

Detailed statistical analysis showed that 97.37% of the variability in streamflow downstream of Ashton Dam is due to variability in inflow to Ashton Reservoir, reflecting flow adjustments at Island Park Dam, runoff from rainfall, the macrophyte-driven cycles, and effects of ice formation. About 1.78% of the variability in streamflow downstream of Ashton Dam is due to what we call “persistence” or “autocorrelation” in the time series. Statistically, the maximum persistence time is one hour and 15 minutes, meaning that flow tends to be relatively constant within any given 75-minute period and changes detectably only when observed over time periods longer than 75 minutes. In more plain terms, if flow is 1000 cfs now, it is very likely to stay near 1000 cfs for the next 75 minutes. Except immediately below Island Park Dam, where adjustments can and do affect streamflow nearly instantaneously, streamflow in the Henry’s Fork, whether upstream or downstream of Ashton Reservoir, does not change very much within any given one- or two-hour period.

Another 0.76% of the flow variability at Ashton Dam is due to random, unmeasured differences between outflow and inflow. These differences are created by effects of inflow from springs and tributaries to the river and reservoir between the Marysville and Ashton gages, irrigation pumping from this same reach, wind waves on the reservoir, reservoir evaporation, and other, obviously minor hydrologic processes.

This leaves us with the remainder of the variability—about 0.09%—that is due to systematic differences between outflow and inflow. Yes, this really is 9/100 of a percent, or 0.0009, attributable to dam operations. This systematic difference between outflow and inflow is a daily cycle in which river stage below the reservoir is, on average, about 0.2 inches lower than that above the reservoir at 4 a.m. and about 0.2 inches higher at 4 p.m. Compare this 0.2-inch amplitude to the 2-inch amplitude of the macrophyte-driven cycles. At the Ashton gage, a 0.2 inch change in stage is equivalent to a change of less than 30 cfs under late-summer conditions, 3% or less of the river’s flow.


The statistical evidence overwhelmingly shows that the Ashton power plant is operated in run-of-river mode. Over 97% of variability in outflow is reflected by inflow, as measured 1.6 miles upstream of the reservoir. Most of the remaining 3% of variability is due to hydrologic processes, including persistence in flow, that occur in the 6.7 river/reservoir miles between Marysville and the USGS Ashton gage. Only 0.09% of the variability in flow downstream of Ashton Dam is attributable to systematic daily differences between outflow and inflow, and these differences are reflected in mean daily fluctuations in river depth of plus-or-minus 0.2 inch.

One other conclusion worth mentioning is that without HFF’s commitment to collecting and analyzing the data needed to answer a specific question about the river, we would not have an objective and quantitative answer—only speculation. This is just one tiny little piece of information we are going to gain from our sonde network over the next decade or two. As we continue to collect information, we will be sharing it with other watershed stakeholders to determine how it can be used to improve management of the river for the benefit all river resources, including wild trout.