Post "the Big Flood"

[Castuserraticus]

Life interferes with being able to spend a lot of time on the river. So, my observations are limited to 4 outings at 4 different sites.

 

I think we're going to see that last year's flood changes the river for many years. The finer gravel is gone - blown downstream. Coarser cobble to boulder gravel dominates the sections I fished. Most of the bottom structure, bars, and small channels that provided the desirable historic structures and current breaks were there because of bedforms created by finer gravel bars - longitudinal (parallel to current) and transverse (across current). There are likely internet resources to research for those who want more info. Gravel bars are mobile bedforms. The upstream side is gradual and erosional and the downstream is steeper and depositional. During normal high flow events bars will migrate down stream relative to the strength of the flow. Some bars will remain stable for years - until an event larger than the one that created them mobilizes the gravel again. This is the nature of a freestone river. In their natural state they can be very poor fish habitat. The James River and many sections of the Elbow have highly mobile channels in relatively wider sections of their valleys.

 

The Bow is no longer a true freestone river. The upstream dams have eliminated the influx of new gravel. If you could view an unconfined Alberta freestone river over time, you would see an amazing process. Highly angular blocks enter the headwaters and work slowly downstream, getting rolled, banged, and broken -getting smaller and losing their angularity. As the flow gradient drops and the channel widens, the gravel becomes organized into bedforms. Over many years, you would be able to watch individual gravel bars migrate downstream in a steady parade - forming, dissipating, and re-forming. As long as there is a source of new rocks entering the headwaters. This is one reason why the Bow at Canmore is poorer habitat - relatively unstable environment. Geologists study these processes on a small scale in man made flow flumes.

 

Flowing water is fantastic at sorting. The highest energy flows can move house size blocks a few centimeters to meters by undercutting them. River gradients in Alberta decrease away from the mountains. The river flow will roll rocks along the bottom until the velocity drops below a critical level. Fine mud drops out of suspension in only the quietest eddies and back waters. The flood's flow through the Policeman's Flats area blasted away the vast majority of the finer gravel and left a boulder field. A friend reported to me there are extensive longitudinal gravel bars above Carseland. The artificial weir created a small area of decreased gradient and flow.

 

The issue I see is, there is no source of fine gravel from upstream to rebuild the in-stream structure. The Highwood is the only mountain sourced tributary that will be able to contribute new gravel. The only other source of finer gravel is local (intra-basinal) - cannibalizing the banks. This makes the river wider and shallower. The river bed is now built of boulders. There are still pockets along the bank but there doesn't seem to be mid stream structure. The finer gravel bars tend to have relatively steep downstream sides making for nice pockets. The boulder fields are more sheet like.

 

I understand the processes and believe I see consequences. Can others confirm the observations? is there some reduction in fishing quality?

 

 

[fishinglibin]

I was out with a biologist the other day, and he lived here most of his life.He does lots of water work in BC on all flowing. He feels there is a diff for the worse and like you found the bottom to be extremely changed and commented on lack of smaller gravel and to much silt almost every where. Lets see what the next couple of run offs brings as far as removing some silt and moving gravel. PS it is not as though he was in panic mode. He was expressing personal opinion on his observations. PL

[DonAndersen]

Cast....

 

The same thing happened in the Red Deer below the dam. All that is left is cobble. The Livingstone within the "gorge" section has little small gravel left due to flood velocities.

Where it can, both rivers have straighter sections with higher velocity flows leading to extensive tailouts. The tailouts change to channels to the next pool/run. This behaviour existed in the Oldman above the dam after the flood of '95.

I think your concern about fines is bang on. What will rebuild the banks leading to decent habitat?

 

Don

[Castuserraticus]

The situation will change when the reservoirs completely fill with sediment and the dams are torn out. Engineers have likely done the calculations. The Elbow River adds to the Weaselhead delta and lake bottom sediment every year and the Glenmore reservoir gets smaller. It will be decades. I've read notes about several old reservoirs in the US that filled and the dams were then destroyed.

 

Nothing can economically change the situation. It will be interesting to see how the fishery changes.

[DonAndersen]

Cast...

 

The Bassno Dam on the Bow was nearly full of silt 50 years ago. It is still there so the irrigators get their fix of water. Electrical Dams will go, irrigation - not a chance.

 

Don