As the dam removal projects edge closer I decided to look at the most reliable source for data I could find - the USGS - to see if I could predict what the river will be like if this project goes forward. Right now we have a broad (or broader in some places) that seasonly fluctuates in height as much as 7 ft. - from low water when the gage is reading between 3 and 4 ft through various stages of flooding at 708-9 and, thankfully not very often, almost to ten feet.
When you look at any body of water, you can't tell much about how deep it is. Aesthetically, it's the width, the breadth, what it covers and what it doesn't that makes the visual impression. Since the river varies in elevation, water, because of gravity, flows from higher ground to lower ground, from upstream to downstream, in our case from north to south. So generally the minimum channel elevation, that is, the bottom of the river, is at a higher elevation upstream than downstream. There are some bumps and holes, but generally that's the case, and generally the river flows below its banks which is why it stays there and doesn't spread out.
At times of flood, it may rise higher than the containing banks in some spots and indeed spread out quite far. It usually goes only as far as the common elevation can be sustained by the volume of water in the river, but sometimes the water is pushed over some part of the land when it is high and then remains trapped there when ther river recedes. Depending on things like the kind of land, the temperature, etc. it may stay in some places, sitting stagnant for weeks and even months at a time, making a perfect place for mosquitos to breed, vegetation to rot and the ground to remain marshy, muddy and messy. This happens all along the river in areas like Zoo Woods, along the first Avenue side across from Bloomingbank, up around Indian Gardens, in Riverside Lawn, the Swan Pond, etc., all the way down the river through Lyons where I took these pictures of what the river is like below the dam, which is a good indicator of what it will be like in Riverside if it is removed.
I predict that large swaths of the river bed which are now more than 100 ft across and look like a "real river" will, for months of a time, shrink down to less than 50 feet wide, be very shallow and unusable for fishing or boating, and expose large, uneven, muddy, sediment coated banks where water will collect in the low spots. According to the USGS data I downloaded, the mean of monthly water height at the gage for the years 2000-2009 is below 3.9 ft. 9 out of 12 months of the year. In 65 of the 99 months for which there was data - meaning for two-thirds of the year these conditions will prevail all along the Des Plaines river through Riverside. Of course, there won't be any plants or trees growing there as there are in these pics of the inundated areas in Lyons because our problems will be out in the actual present river bed itself, which will periodically flood and wash them away as it does now. Indeed, there won't be much of anything out there except rocks, mud and the debris that was dumped out there over the last hundred and fifty plus years. The scenic beauty of Riverside's river scape is dependent on the dams, which have been here in some form for about 184 years. That's why the Riverside Improvement Company bought this land. Remove them, and see what you get. But do so at your peril.
Let me point something else out - using the Army Corps own numbers: The crest of Hofmann dam is at 605.04 ft elevation, and they say it backs up 156 acre feet of water. I know the height is accurate, the other is sort of a mystery number they get from somewhere, but, since we agree water flows downhill and always seeks its own level, it would be reasonable to assume that the backed up water in the river channel behind Hoffman extends at least to the first point that is higher than Hofmann's crest. That is not at the railroad bridge, or Forest Avenue as they have stated, but way up north. Remember that for a moment.
Now the base of the Hofmann Dam is at elevation 592. - 594. (it seems to slant a bit) and the Corps generally uses 593.6 ft. for theiir measurements. So the dam is, in round numbers, 12 ft. high, and if/when they cut a 150 foot wide notch through 60% of the dam down to bedrock they will lower the surface of whatever amount of water runs over the dam - which is about 3-6" when the gage is in the 3 ft. range. So at times other than floods, the water you see at the dam will be "about" 12 ft. lower than it is now in the river channel. That's it, down there, that little stream that looks like Salt Creek.
I say "about" because if, as when I measured it on October 28th, 2010, and the gage was reading 3.21 ft. there was about 3" of water flowing over the dam crest from end to end, which we know is 258 feet long. Take that amount of water and run it through a 150 foot wide channel and it will give you a river depth .043 feet. Depending on the configuration of the bottom, how level it is, etc., you can have, for maybe nine months a year, a river 150 ft. wide and less than half a foot deep or one 66 ft. wide and .9 feet deep or one 33 ft. wide and 1.95 ft. deep, but you can't have what you have now.
Of course, since the water upstream will be flowing out through the "notch" the river will be lowered in depth all the way upstream far beyond Riverside, because the new minimum channel height will no longer be 605. ft elevation, but rather 593.6 ft. elevation. Everything upstream all the way to the Armitage dam is above that elevation, so that water will also pour out through this "notch", and only the sustained (replenished) flow of water coming in will make the "new" river. At periods of low flow that gets down to about 3 inches as we have seen - the Corps writes it as .24 feet - so that is what you will have - and all you will have - flowing through the river all through Riverside. And that will expose large, muddy, smelly, debris-laden banks on both sides as far as the river is narrowed. And make no mistake, the Corps is not going to come in and create some water wonderland or paradise on earth wetland anywhere but a short distance above the dam. Anyway, whatever is planted out there in the "new" river bed will be subject to the same level and duration of flooding and current as it has now - this project has no effect on higher levels of flow. Above where the Corps will work, roughly where Coonley Road meets Fairbank Road, you're on your own.
Don't worry, you'll know when this happens. You'll be able to smell it.
Of course, when Fairbank is removed, the water backed up under Millbridge to the base of Hofmann dam will be gone, and that area, and way down to the Swan Pond and beyond, will also be exposed in a similar way. Already, in summer/fall, you can almost walk across the river below Fairbank, and I've posted pics on the front page of this blog that show that condition, there, and what it looked like below the old road bridge before the present dam was built. That's visual proof of what this will all look like if we let it happen. You can also see in the darker picture with the Tower that at times there is not even enough water flowing to completely cover little Fairbank Dam - see there on the right the part that is sticking up. I have seen 8-10-12 feet - up to 10% of its length, uncovered for weeks at a time. Don't let them turn this beautiful, tranquil spot just tn miles from the Sears Tower into a rock quarry.