Summary

At the request of the Kootenai Tribe of Idaho, the U.S. Geological Survey (USGS) extend a two-dimensional flow model of the Kootenai River upstream into a braided reach that is upstream of the present-day white sturgeon spawning reach. Many scientists consider the braided reach a suitable substrate with adequate streamflow velocities for re-establishing recruitment of the endangered white sturgeon and that extending a model upstream will be helpful in assessing the feasibility of various strategies to encourage white sturgeon to spawn in this reach. At the request of the Idaho Department of Fish and Game (IDFG) the USGS also extended the older two-dimensional flow model several kilometers downstream of the white sturgeon spawning reach. This modified model can quantify the physical characteristics of a reach that white sturgeon pass through as they swim upstream from Kootenay Lake in British Columbia, Canada, to the spawning reach near Bonners Ferry, Idaho. The USGS Multi-Dimensional Surface-Water Modeling System was used for the initial modeling effort and for this subsequent modeling effort.

Two two-dimensional flow models of the Kootenai River braided, straight, and meander reaches form one contiguous 31.8 km model-simulated reach from river kilometer (RKM) 222.2 to 254.0. The model of the braided reach extends from RKM 242.9 to 254.0 and includes the 2.8 km straight reach and a 1.6 km segment of the meander reach and is referred to as the braided-straight reach model. The previous USGS two-dimensional flow model of the meander reach includes the straight reach and is extended 6.2 km downstream; the model reach is from RKM 222.2 to 245.9 and referred as the meander-straight reach model. The computational grid used to model the Kootenai River braided, straight, and meander reaches forms an approximately 10 × 10-m grid.

The braided-straight reach and meander-straight reach models were run over a range of streamflows to link physical characteristics of streamflow to biological or other habitat data. The models were used to simulate water-surface elevations, depth, velocity, and bed shear stress in the Kootenai River for streamflows of 170, 566, 1,130, 1,700, and 2,120 m³/s. The range of simulated streamflows was selected to span typical river conditions before and after the construction of Libby Dam. The highest simulated streamflow (2,120 m³/s) represents a discharge that is approximately equal to the annual median peak streamflow (2,240 m³/s) prior to emplacement of Libby Dam in 1972. The water-surface elevation at the downstream boundary of both models was set for each streamflow condition using a one-dimensional model. The discharge for each streamflow condition corresponds to the median water-surface elevations in Kootenay Lake at the Queens Bay, British Columbia, Canada, (08NH064) gaging station.

Periodic updates to the braided-straight reach model are required to continue to accurately simulate hydraulic conditions in the braided reach into the future. The basis for this requirement is that bathymetry input into the braided-straight reach model will, over time, misrepresent that reach because of shifting channels, scouring, and deposition. Therefore, the reach must be mapped periodically, the resulting bathymetry data input into the model, and the model recalibrated. Additionally, calibration of this model uses stream discharge measured at the Tribal hatchery gaging station that is computed using an index-velocity ratio. In the future, an adjustment may be applied to the computation that causes a shift in discharge over certain ranges of streamflow. This adjustment would take into account that the river has a mobile bed, and that changes in bed elevation across the channel at this gaging station may occur between low and high streamflow. As additional bed-elevation measurements are made at the gaging station and are evaluated for effect on flow, that information may be included in the discharge computations, and cause a shift in discharge over certain ranges of streamflow. Therefore, the model will need to be recalibrated to reflect this shift in discharge.

The braided, straight, and meander reaches can be divided into four distinct velocity zones on the basis of simulated maximum depth-averaged velocity; moreover, an analysis of simulated velocity indicates that white sturgeon may be focusing on a specific velocity signature. Zone A, the main channel of the braided reach, is a region of generally high velocity except near RKM 248. Zone B, the straight reach, is a transition zone from the high velocity braided reach to the low velocity meander reach. Since 1994, IDFG has detected less than a few percent of the overall white sturgeon spawning in zone B. The contact between zone A and B is variable due to changing backwater conditions over the range of the streamflows. At streamflows greater than about 566 m³/s, backwater conditions extend upstream more than 1 km into the lower part of the braided reach resulting in decreased velocities that are somewhat similar to velocities in the straight and meander reaches. Zone C, between Ambush Rock and the first sharp meander downstream of the mouth of Deep Creek at RKM 237.5 has uniform maximum streamflow velocities compared to the rest of the meander reach. One notable exception is near the deep scour hole and lateral recirculation eddy upstream of Deep Creek. Less than 15 percent of the overall white sturgeon spawning has been detected in zone C. Maximum velocity increased with increasing streamflow and formed distinct areas of relatively high velocity in zone D, downstream of zone C and extending to the downstream model boundary at RKM 222.1. These areas of relatively high velocity and white sturgeon spawning locations have been correlated. Studies on the Columbia River indicate that sturgeon seek areas of high velocity for spawning. Most white sturgeon spawning in the Kootenai River has been observed in zone D. Spawning locations and simulated depths indicate that sturgeon may avoid spawning in the braided reach due to shallower water.

Streamflow velocities equal to or greater than 1.0 m/s are likely to greatly reduce or eliminate predation of white sturgeon eggs. Simulated depth-averaged velocity in the meander reach does not approach 1.0 m/s until streamflow is about 2,120 m³/s. Based on maximum simulated velocity in the meander reach, velocity in parts of the channel approach 1.0 m/s for streamflows roughly equal to or greater than 
1,130 m³/s. However, velocities in the main channel of the braided reach, generally exceed the 1.0 m/s threshold.

Flow structure at the white sturgeon spawning locations in the meander reach differs substantially from the flow structure in the braided reach. This difference could have some influence on the location of spawning events. Relatively high velocity and white sturgeon spawning locations have been correlated. These areas of relatively high velocity occur in the meander reach at the same locations regardless of the discharge magnitude as modeled over a range of pre- and Libby Dam era flow conditions. In the braided reach, locations of regions of higher velocity are variable and shift on the order of kilometers in the upstream or downstream direction under different backwater and streamflow conditions. This flow structure behavior can be attributed to the single-threaded meander reach having uniform channel geometry compared to the multi-threaded braided reach. As stage declines or the streamflow decreases to less than about 1,130 m³/s in the braided reach, the various minor channels and sloughs of the river begin to dewater causing the regions of higher velocity to change location. It is not known if white sturgeon can perceive the evolving velocity structure of the braided reach as too complex for spawning site selection.

Output from the braided-straight reach model was used to report on the length and percentage of longitudinal profiles of the Kootenai River meeting the U.S. Fish and Wildlife Service Biological Opinion minimum criteria for depth and streamflow velocity during the May and June white sturgeon spawning season. During peak-flow augmentation the BiOp specifies a minimum depth criteria of 5 to 7 m or greater for 60 percent of the thalweg longitudinal profile between RKM 244.6 and 252.7. This habitat includes parts of the braided and straight reaches. The 5-m depth criterion for the braided-straight reach longitudinal profile was met every day from May 18 to June 5, 2006, except for June 1 when streamflow was the lowest (986 m³/s). Mean daily streamflows from May 18 to June 5, 2006, ranged from 986 to 1,300 m³/s, and the percentage of the longitudinal profile meeting the habitat criterion ranged from 58 to 91 percent. Although the 5-m depth criterion was met during 2006, only 17 percent of the tagged fish went upstream of the Route 95 Bridge and 7 percent went upstream of RKM 246.6 into the multichannel braided reach, and spawning was still recorded downstream in the meander reach. During May 22 to June 5, 2007, the braided-straight reach failed to meet the 5-m depth criterion. Mean daily streamflows during this period ranged from 878 to 980 m³/s, and the percentage of the thalweg longitudinal profile exceeding the water depth criterion ranged from 46 to 59 percent. During 2006 and 2007, the percentage of the thalweg longitudinal profile meeting the 7-m habitat criterion ranged from 10 to 44 percent. The streamflow velocity criterion was met each day during June 5–30, 2006 and 2007. Percent of the maximum-velocity longitudinal profile of the river meeting the 1 m/s velocity habitat criterion during this period ranged from 70 to 96 percent. The percent of the maximum-velocity longitudinal profile meeting the velocity criterion varies for a specific streamflow due to the day-to-day variable location of the transition zone between the free-flowing river and backwater from Kootenay Lake in the braided reach.