Boise Valley Fly Fishers has wrapped up their Brown Trout Redd mapping on the Lower Boise River and delivered updated waterproof maps to Boise Flood District 10 for use in their annual winter river maintenance. Now that the redd map is complete we plan to refloat the river yearly to identify which sites are active and communicate that to the Boise Flood District and Idaho Fish and Game. Last year the Boise Flood District reported that the redd mapping had enabled them to avoid all but one of the redd areas and the Flood District excavator driver, shown in Figure 1 holding the Redd Map along with new Assistant District Manager Mark Zirschky, told me that he has also been able to identify and avoid smaller redd areas that were not on the map. Well done and thank you to the Boise Flood District!
BVFF is tracking 50 redd sites across 22 miles of the Boise River, from the Willow Lane boat ramp in Boise down to Star. This year we were able to add the South Chann el around Eagle Island, which had fewer redd sites than the North Channel, but some of the sites had quite large gravel deposits and many brown trout redds. Figure 2 shows the area of the Boise River that we are tracking, and you can see how many more redd sites are in the North Channel vs the South Channel. There are many factors that could be responsible, but one reason is the North Channel has more “sinuosity” (a measure of the number of curves in a river channel) resulting in more areas where smaller gravels settle out that trout can use to spawn.
This year spawning redds were observed as early as the first week of November and as late as the 2nd week of December. Spawning locations were consistent between 2021 and 2022 but this year we counted fewer total numbers of redds. This is partially due to being more conservative in our counting and not including some areas that IDF&G fish biologist Tim D’Amico thought could be caused by scour from higher velocity flows, but even then the overall redd numbers appear to be down. On a positive note, the number of redds in our BVFF Side Channel increased this year because of gravel shifting into prime locations and having a little more water flowing into the channel. Idaho Fish and Game is very interested in tracking the redd locations from year to year and we will continue to report our findings to them.
This year we piloted a process of noting the location of downed trees as we floated the river, to help the Boise flood district plan their winter maintenance work, which has opened the door for more discussions with the Flood District and Idaho Fish and Game on identifying trees that would be good to try and retain for trout fry protection. A great continuation of our partnership on the Boise River.
Last year we floated in November and missed some of the brown trout redds because browns were not done spawning so this year we waited until December to make sure that brown trout spawning activity was complete. Next year we will switch to checking known redd-zones for activity which should allow us some weekend outings in November. Thanks to our volunteers (George Chen, Klaus Kissman, George Butts, Johnny Rogers, and Troy Pearse) who braved cold days on the river to count redds.
