The purpose of the flushing flow release from Echo Dam, as stated by the Utah Division of Wildlife Resources (UDWR), is to remove fine sediment (i.e., sediment <2mm in diameter) from the bed of the Weber River in a reach extending from the intake of the Stoddard Diverson to the USGS gage on the Weber River at Gateway, UT (USGS gage 10136500), a length of ~10 miles. A secondary objective, also articulated by UDWR, is to remove vegetation from the river bed.

In collaboration with UDWR and Trout Unlimited, we will conduct monitoring before, during, and after the flushing flow event. The purpose of the monitoring is to determine whether the flushing flow met the management objectives and to develop a long-term monitoring protocol for future flushing flows. The monitoring protocol described here will address the following questions. a) How much fine sediment and vegetation was removed from the channel bed? b) How long after the flushing flow does it take for fine sediment and vegetation to reestablish?

Currently, the past few years of drought have not provided any runoff flows to remove the fine sediments, gravel, and cobbles. Additionally, the macrophyte vegetation growth is increasing each year. If this flush does not occur, would continue to impact the fishery in this reach with high water temperatures and low oxygen levels.

Bonneville cutthroat trout Conservation Agreement and Strategy; Three Species Conservation Agreement and Strategy

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The partnership of the groups involved have agreed that this opportunity will allow us to learn what flows are needed to clean this reach of the river in times of excess water upstream during the winter. This will hopefully increase water quality in the futre.

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We will employ some simple methods including painting point bars with bright spray paint, collecting pebble counts, monitoring and collecting turbidity measurements during the flows, creating and maintaining photo points and possibly using a drone to monitor growth of vegetation.

To answer these questions, we propose the following monitoring activities. I. Bed mobility study to determine the grain size of gravel mobilized during the flushing flow. Paint gravels of different grain sizes in 5-10 monitoring sites throughout the study reach and determine if the gravel bed moved enough to allow fines to be flushed. II. Pre- and post-surface measurements at 5-10 monitoring sites throughout the study reach before the event, directly after the event, and every 3 months for the following year. III. Pre- and post-flushing flow photography of bed vegetation at 5-10 monitoring sites throughout the study reach before the event, directly after the event, and every month for the following year during the growing season. IV. Suspended sediment concentration sampling (work contracted to USGS) OR turbidity/LISST measurements (completed by USU or Trout Unlimited) during the flushing flow event. Possible discharge measurements from Peterson Bridge if Stoddard Diversion is unmeasured. V. Analysis of in-channel vegetation removal and channel adjustment caused by the flushing flow event. Pre-event images from 2021 NAIP and post-event satellite images from Planet Labs (application for an academic license pending).

Utah State University Weber Basin Water Conservation District Trout Unlimited UDWR PacifiCorp Weber River water commissioner

Based on the flows and data collected, UDWR, TU, and USU will use the data for future flows and what flows are needed and for how long. With this information, UDWR and TU will coordinate with Weber Basin for additional releases if needed

This flushing flow will allow better water quality and quantity in this reach during the winter season. It will maintain a quality fishery that struggles during the summer with low flows, higher water temperatures, and low oxygen levels. The vegetation may add oxygen to the water for the fishery but also slows flows and may elevate temperatures.

02/01/2023

06/30/2023

2023

In collaboration with Utah DWR, WBWCD, and TU, a flushing flow experiment and monitoring program was planned for early spring 2023. Echo Dam releases were increased to greater than 1000 ft3/sec before the snowmelt runoff to ensure fine sediment transport measured at the Gateway gage was from in channel sources (i.e., flushing fines) rather than tributary supply. Monitoring took place in the vicinity of the Gateway gage. The USGS Utah Water Science Center conducted all fieldwork and sampling. A turbidity probe was installed at the Gateway gage on March 13th, 2023, and remained operational through the 2023 spring flood, measuring turbidity at 15-minute increments. Turbidity measurements provide a continuous record of the optical properties of the water through time; however, turbidity is not a direct measure of suspended sediment concentration. Therefore, turbidity measurements were supplemented with direct suspended sediment concentration measurements using depth-integrated samples collected at equal discharge increments and pump samples. The Equal-Discharge-Increment (EDI) samples were collected with a DH-95 sampler from the USGS cableway at the Gateway gage at five sampling stations, determined by concurrent discharge measurements. Five EDI samples were collected: three samples were collected during the flushing flow event, and two samples were collected during peak flow in the subsequent snowmelt runoff event. The EDI samples are being processed at the USGS Cascades Volcano Observatory for suspended sediment concentration and percent sand and silt/clay. ISCO 6712 auto pump samplers were installed on March 13th, 2023, at (1) the Gateway gage adjacent to the turbidity probe and (2) 200 ft directly upstream from the Gateway gage. Pump samples were collected approximately every 6 hours through mid-April, every 12 hours from mid-April to early May, and daily from early May to June. The pump samples are being stored at the USGS Utah Water Science Center, pending further funding for suspended sediment concentration and grain size analysis. A reference reach for monitoring regrowth of aquatic vegetation was established at Red Barn Angler Access and high-resolution aerial imagery was collected by Trout Unlimited. This imagery will be analyzed with pixel classification methods and can be compared to future imagery using these methods.

The Weber River basin is heavily developed, with 7 major reservoirs and three diversion supplying water for irrigation and consumptive water uses along the Wasatch Front (Kraft, 2017). Echo Dam is the primary engineering structure on the Weber River, constructed in 1931 as part of the Weber Basin project to store snowmelt runoff for agricultural irrigation and consumptive water use. Echo Dam releases elevate summer base flows and reduce snowmelt floods in the spring. Most of the summer base flow is diverted into the Gateway Canal at the Stoddard Diversion, located approximately 30 miles downstream from Echo Dam. The Gateway Canal delivers water to the Wasatch Front through the Weber and Davis Aqueducts. A proportion of flow diverted at Stoddard is returned to the Weber River through a hydropower structure located approximately 450 feet upstream from the Gateway gage (Weber River at Gateway, UT 10136500). Echo Dam releases and flow diversions have the greatest effect on Weber River discharge in the reach between the Stoddard diversion and return flow point upstream of the Gateway gage, we refer to this as the Peterson Reach. Channel conditions and habitat are degraded in the Peterson Reach during consecutive dry years as sediment accumulates during the summer months. Approximately 80% of the summer base flow is diverted into the Gateway Canal at Stoddard, causing the average summer flow in the Peterson Reach to be less than a quarter of flow passing the Gateway gage (67 ft3/sec ± 58 ft3/sec compared to 306 ft3/sec ± 91 ft3/sec in the Peterson reach and Gateway gage, respectively). Reduced summer flows do not have capacity to transport the sediment supply, causing fine sediment to accumulate on the gravel bed and an overgrowth of in-channel aquatic vegetation, choking the low-flow channel. During wet years, spring releases from Echo Dam and large tributary inflow mobilize the gravel bed, flush fine sediment, and reset aquatic vegetation growth. During dry years, runoff storage in Echo Reservoir dampens the downstream flood, which limits how much fine sediment is removed from the Peterson reach, and the channel becomes more impaired the following summer. Consecutive dry years, such as 2020 to 2022, are particularly problematic, because large amounts of fine sediment progressively accumulate on the channel bed, degrading in channel habitat. Modifying spring releases from Echo Dam during consecutive low flow years to flush fine sediment could help reverse degraded conditions in the Peterson reach. In this study, we estimated the discharge to flush fine sediment from the Peterson reach and evaluated the effectiveness of our discharge estimate during a flushing flow experiment. The flushing flow was estimated as the discharge needed to mobilize the gravel bed in the Peterson reach using principals of open channel flow and sediment transport. Then, we implemented a flushing flow experiment during spring 2023 in collaboration with the Utah Department of Natural Resources, DWR, WBWCD, USGS, and Trout Unlimited. A report detailing the findings is pending. However, the future management section of this completion report details recommendations based on the preliminary findings.

The early-spring natural pulse event flushed most of the in-channel fines from the Peterson reach. We recommend that future flushing flows have a discharge in the Peterson reach exceeding 1000 ft3/sec to flush fines from the channel bed and reset aquatic vegetation growth. During the early-spring natural pulse flow, Peterson reach discharge slightly exceeded our 1000 ft3/sec flushing flow recommendation, and turbidity measurements indicate that this flow depleted fines from the channel bed. Turbidity measurements dropped to background levels after three days, which suggests that future flushing flows should occur for a minimum of three days. All of our interpretations use turbidity as a surrogate for suspended sediment concentration. While suspended sediment concentration strongly affects turbidity, other optical properties of the flow also influence turbidity, which could complicate our interpretation of the turbidity record presented in this report. For example, coarse grains produce less scattering, so turbidity decreases as the suspended sediment load coarsens (Voichick and Topping, 2014). The finest grain sizes (i.e., silt and clay) are carried high in the water column and move downstream the fastest. As a result, suspended sediment is typically finer early in the flood and coarsens as the flood progresses. Early in the flood, turbidity may increase rapidly as slit/clay arrives at the monitoring station. Later in the flood, turbidity may drop as coarser grains arrive at the monitoring station, even if the suspended sediment concentration remains constant or increases. Fully interpreting the turbidity record at the Gateway gage will require a better understanding of grain size changes in the suspended sediment concentration and its effect on turbidity. We assumed turbidity decreased during the early-spring natural pulse flow as in-channel fines depleted. There is the potential that the suspended sediment concentration coarsened instead, causing turbidity to decrease before the in-channel fines were flushed. Similarly, turbidity during the early-spring pulse flow was significantly higher than the flushing flow experiment, despite the discharge during the flushing flow experiment being 33% greater. We attributed the decrease in turbidity to a reduction in within-channel fine sediment supply. However, the supply grain size could have been coarser during the flushing flow experiment because the discharge was larger, entraining coarser sand higher in the flow. Thus, a proportion of the reduction in turbidity during the flushing flow experiment could have resulted from supply coarsening, and the suspended sediment concentration could have been relatively high. Future work should focus on evaluating the effect of the suspended load grain size on turbidity. The pump samples collected during the early-spring pulse flow and flushing flow experiment provide an opportunity to evaluate how the load's grain size changed throughout these events. Suppose the suspended load during the early natural pulse flow was substantially finer than the flushing flow experiment. In that case, there is the potential that the flushing flow was more effective at removing fines than presented in this report. The total mass of fine sediment removed during the flushing flow experiment could be estimated by relating the EDI samples to the turbidity measurements if the concentration grain size did not change significantly during the event. Alternatively, the supply grain size may be of little concern if the purpose of the flushing flow is to remove only silt/clay, because supply coarsening and reduced concentration would both indicate silt/clay was flushed from the bed and cause turbidity to decrease. Secondary to sediment transport efforts, Trout Unlimited has committed to collecting future aerial imagery for the purpose of monitoring regrowth of aquatic vegetation.