Percentage of Braided and Straight Reach Meeting Habitat Criteria During White Sturgeon Spawning Season

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 2006 Biological Opinion (BiOp; U.S, Fish and Wildlife Service, 2006) minimum habitat criteria for depth and streamflow velocity. Streamflow in the Kootenai River is augmented with water released from Libby Dam to benefit of white sturgeon migration, spawning, and downstream dispersal during the early life stages. This streamflow augmentation is summarized here and details are included in the 2006 Kootenai River BiOp (U.S. Fish and Wildlife Service, 2006), which addressed how the U.S. Army Corps of Engineers and the Bonneville Power Administration operate Libby Dam in Montana and the effect of dam operation on the endangered Kootenai River white sturgeon and its critical habitat. The BiOp specifies a minimum depth of 5–7 m or greater for 60 percent of the thalweg longitudinal profile between RKMs 244.6 and 252.7 during peak-flow augmentation. This habitat includes parts of the braided and straight reaches. Peak-flow augmentation is based on a full-powerhouse discharge of about 708 m³/s at Libby Dam, which generally occurs in mid–late May and early June during the white sturgeon spawning season (U.S. Fish and Wildlife Service, 2006). The depth criteria were established to ensure adequate depth for white sturgeon to swim upstream from the sandy meander reach into the braided reach and spawn where gravel and cobble substrate conditions are favorable for the survival of sturgeon eggs. Currently, less than 30 percent of sturgeon swim upstream into the braided reach (Pete Rust, Idaho Department of Fish and Game, oral commun., 2008). The depth criteria were evaluated by analyzing simulated depth from the braided-straight reach model for the period of peak flow augmentation during May 18 through June 5, 2006, and May 22 through June 5, 2007.

The BiOp also specifies a minimum streamflow velocity criterion of 1 m/s in 60 percent of the maximum-velocity longitudinal profile between RKMs 244.6 and 252.7 during post-peak flow augmentation, for as many as 21 days. Post peak-flow augmentation occurs in mid–late June and early July during the white sturgeon spawning season. The velocity criterion is intended to provide large enough streamflow velocity to increase the likelihood of white sturgeon recruitment. Larger velocities reduce the ability of other fish species from preying on sturgeon eggs and aid egg incubation and downstream dispersal of sturgeon during early life stages. The streamflow velocity criterion was evaluated by analyzing simulated velocity from the braided-straight reach model for the period of post-peak flow augmentation on June 6–30, 2006 and 2007.

The thalweg longitudinal profile and the maximum-velocity longitudinal profile are in different locations. The thalweg longitudinal profile was fixed over the range of streamflows and the maximum-velocity longitudinal profile shifts laterally with different streamflows and backwater conditions in the multi-threaded braided reach. Backwater conditions in the braided reach are a function of streamflow and Kootenay Lake water-surface elevation. Day-to-day variability can occur in the location of the transition zone between the free-flowing river and backwater from Kootenay Lake. When the river is within its banks, streamflow velocity increases with increasing streamflow in the absence of backwater. However, when the river changes from an absence to a presence of backwater the streamflow velocity tends to decrease.

The braided-straight reach model was run for each day by specifying the mean daily discharge measured at the Tribal Hatchery gaging station (12310100). Mean daily water-surface elevations for the downstream model boundary were computed by interpolating measured water-surface elevations from the Bonners Ferry (12309500) and the Tribal Hatchery gaging stations.

The longitudinal-probe tool available in MD_SWMS was used to extract simulated depth for every model node along the thalweg longitudinal profile. These data were used to compute the length and percentage of the river’s longitudinal profile that were within the depth criteria identified in table 6.

MD_SWMS was used to develop maps of simulated depth-averaged water velocity to evaluate the velocity criterion. A mapping approach was used to extract velocity from model simulations because the location of the maximum-velocity longitudinal profile shifts laterally with different streamflows and backwater conditions. Color-coded maps of simulated water velocity with 0.10 km markings along the river were used to evaluate the velocity criterion along a maximum-velocity longitudinal profile in the critical habitat reach. Data from these maps were manually extracted to compute the length and percentage of the river’s maximum-velocity longitudinal profile in compliance with the velocity criterion (table 6).

The 5-m depth criterion for the braided-straight reach longitudinal profile was met every day from May 18 to June 5, 2006 (table 6) except for June 1 when streamflow was the lowest at 986 m³/s. Mean daily streamflows from May 18 to June 5, 2006 ranged from 986 to 1,310 m³/s, and the longitudinal profile meeting this habitat criterion ranged from 58 to 91 percent. 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 multi-channel braided reach, and yet spawning was still recorded downstream in the meander reach. During May 22 to June 5, 2007, the braided-straight reach did not meet the 5-m depth criterion (table 6). Mean daily streamflow during this period ranged from 878 to 980 m³/s, and the thalweg longitudinal profile exceeding the water depth criterion ranged from 46 to 59 percent. During 2006 and 2007, the thalweg longitudinal profile meeting the 7-m habitat criterion ranged from 10 to 44 percent. The relation between streamflow and percentage of the thalweg longitudinal profile between RKMs 244.6 and 252.7 that met the water depth criteria during 2006 and 2007 is shown in figure 13. Variability in the data indicates that depth can vary for a specific streamflow. Because depth in the braided reach is a function of streamflow and the water-surface elevation of Kootenay Lake, day-to-day variability can occur in the transition zone between the free-flowing river and backwater from Kootenay Lake.

The streamflow velocity criteria were met each day during June 5–30, 2006 and 2007 (table 6). 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 percentage of the maximum-velocity longitudinal profile meeting the velocity criterion varied for specific streamflows due to the day-to-day variability of the transition zone between the free-flowing river and backwater from Kootenay Lake in the braided reach. Figure 14 shows that as streamflow increases from about 500 to about 800 m³/s the percentage of the longitudinal profile meeting the velocity criterion decreases. This condition resulted from filling of Kootenay Lake, which lead to increased lake stage and backwater conditions that extended father upstream into the braided reach, which then decrease velocity. The percentage of the maximum-velocity longitudinal profile meeting the velocity criterion increases for streamflow greater than about 1,000 m³/s.