The Wild & Scenic section of the Crooked River below Bowman Dam is prized by anglers from all over Oregon who target abundant wild, native mountain whitefish and redband trout. The Oregon Department of Fish & Wildlife annually surveys fish populations in the river in June. Unsurprisingly, over the years ODFW has recorded a direct connection between river levels and fish health. Specifically, flows below 35 cfs have been shown to have strong negative impact on trout abundance.
The period from mid-September to November 1, 2022 witnessed an unprecedented reduction in flows in the Crooked to approximately 10 cfs. In response, ODFW conducted a survey in October to get a preliminary idea of the impact. We really won’t know what has happened until the survey next June, but it should not be surprising that the initial results do not look good.
You can read the entire draft report yourself*, but as predicted by ODFW, and obvious to anyone who has spent any time fishing the Crooked River, it has been dramatically impacted. I’ll get into the details below, but first there is a massive caveat that you must be aware of.
ODFW’s annual fish counting is traditionally done in June. Per the Bureau of Reclamation, average flows in the river that month range from 380 cfs to 240 cfs. Last June the river was at 210 cfs during the sampling period. ODFW floats 2.5 miles of the river in a rubber pontoon boat, shocking part of the river with an electric current, and counts the temporarily stunned fish. (It’s a little more complicated, but that’s a good enough explanation for here.)
Of course, at 210 cfs the fish in the river are highly mobile, can avoid a boat, and are spread out across the width of the river. Sampling efficiency in that environment will be much lower than at 10 cfs when surviving fish are trapped in small pools and essentially all of them will be shocked and counted. Also, the October study was only done over 0.4 miles versus 2.5 miles in June. Comparing sampling results from last June and October must be done with this in mind. As ODFW states in their draft report, we will only understand the impact of the extreme low flows when the annual count happens next June after which a direct comparison between similar studies can be made. Nevertheless, the initial results are deeply concerning.
Instream flow modeling of the Upper Crooked River Canyon Reach below Bowman Dam indicated that at flows of 10 cfs, adult Redband Trout habitat is reduced to less than three percent of maximum available, adult Chinook Salmon spawning habitat is essentially eliminated (<0.1% of maximum) and juvenile Redband, steelhead, and Chinook Salmon rearing is reduced to around 30 percent of maximum (Hardin 1993, 2001, 2011). The decrease in available and suitable habitat under low flow conditions could restrict diel or seasonal movement to important habitats, result in fish congregating in the remaining pool habitats, cause increased stress and disease transmission, and could lead to the immediate or delayed mortality of a large portion of the native fish population. Extremely low flows will also expose cobble and gravel bars reducing benthic macroinvertebrate production, an important forage base for Crooked River fishes. (Bold emphasis added.)
Preliminary results show that whitefish populations have declined by 87%! The decline in the total number of redband trout was 21%, but the surviving fish are almost all under 7.9″ (~200 mm) in length. In addition, surviving trout weights were significantly reduced compared to June, indicating a lack of food. This is hard to understand and no explanation was offered on why these two species of fish that have evolved in the same environment had such radically different results. My speculation is that it has to do with the sampling method, but who knows?
A further reflection of this, and my caveat discussion above, are the confidence intervals shown in the graph on page 8. In essence, population estimates from last June could be off by a much larger percentage than last October. So, the declines in trout populations could be higher, but probably not much lower. Also, the dramatic reduction in habitat for aquatic insects, the primary food source for whitefish and trout, will undoubtedly lead to further declines in fish populations, health, or both.
Finally, it is important consider the discussion of dissolved oxygen. Obviously, fish and aquatic insects need to breath. DO comes from turbulence in flowing water trapping oxygen and from photosynthesis occurring in aquatic plants. The ODFW report shows good DO levels during the day as photosynthesis occurs, but dropping dramatically at night to levels below those needed by fish and aquatic insects, creating a daily “bloom-bust” cycle and increased stress on fish.
*There is an error on page 7 of the ODFW report. It states that redband populations were reduced to 1,6475/fish a mile. ODFW tells me that the correct number is 1,647.