Bridge Height for the Mississippi

[jeanathon]

Is there a minimum bridge height for the Mississippi river like the ICW?

[Jammer]

Short answer is about 60 feet at low water, and 52' at the 2% flowline (meaning that at any bridge there's at least 52' of clearance 98% of the time).

I've sailed, motored, and canoed extensively on portions of the Mississippi from Anoka (above the nominal head of navigation) down as far as Winona, and have been a student of the charts and the water levels for years. I live half a mile from one of the tributaries.

The head of navigation is the now-closed Upper St. Anthony Falls lock in downtown Minneapolis at river mile 853. Controlling clearance is 24.5' above normal pool at the nearby Stone Arch Bridge. While the Upper St. Anthony Falls lock was still open, controlling clearance upstream to the Coon Rapids Dam was about 23'. It has never been possible to navigate above the Coon Rapids Dam from the Gulf, because its construction predates the building of the locks at St. Anthony Falls.

Heading downstream from the Stone Arch Bridge, there is an uncharted overhead crane gantry that has been added to the lower St. Anthony Falls lock that is roughly 30' (I almost hit it once). Below that, there's a former railroad bridge, now used for a pedestrian trail, charted at 39.4'. The Franklin Avenue Bridge at river mile 851.5 is arched and is 55' above normal pool at the highest point of the arch.

Below that point, controlling clearance is 59.6' at Roberts St. in St. Paul with honorable mention to the Ford Parkway brdige that is charted at 55' but appears to be considerably higher in the center of the span.

Below St. Paul, there are a number of bridges charted at 60' above pool, including the Hastings RR draw at river mile 813.7, a lift bridge that is 60' when open, the Prairie du Chien bridge at river mile 634.8 which is 60' from pool, and the I-80 bridge at river mile 495.4 which is 60' from pool.

There is also the Burlington Highway Bridge at river mile 404, which has been recently rebuilt. There is no official published height from pool that I have been able to find, though unofficial sources state 60', and the light list just says 52.62' at 2% flowline.

Once you're below the confluence with the Illinois River, controlling clearance is 66.36' above pool at the New Clark Highway Bridge at river mile 202. That's just above the confluence with the Missouri River. Below that point clearance is above 75' during typical summer/fall conditions.

If you're going up the St. Croix, controlling clearance is 60.4' at the railroad bridge at the confluence with the Mississippi, up to the Stillwater Drawbridge. The drawbridge is charted at 57' above pool but some local resources report 65'. (see https://www.johnweeks.com/bridges/pages/sc04.html). Draft quickly becomes limiting upstream of Stillwater, anyway. I've been up to the head of navigation in a dinghy -- that's the Arcola railroad bridge, and navigation is prohibited upstream due to invasive species concerns.

If you're going up the Illinois River, that's 47' until you get close to Chicago, where there's a former drawbridge (now inoperable or at least untended) at 19'6".

Among local boaters in the Chicago area there are rumors that there are people who know how to open "that bridge" and have done so in recent times. Could be true. Could be urban legend.

[Jammer]

So then the question is how much do you really get. It depends on the year, the time of year, and which pool. It's hard to figure out to the exact foot because of datum differences and the fact that there is some fluctuation in pool height between any bridge and the nearest gauge. Generally the ACOE likes to regulate the water levels so they are at or slightly above normal pool in most of the pools. The river is ordinarily above normal pool in the spring, and can stay there all summer in wet years. I would think that for planning purposes 55' would be a good, practical limit. My mast is 55', and if I take my boat on the Mississippi at some point (by having it trucked the 150' miles from Duluth to Stillwater), I'll use a masttop-mounted backup camera to be absolutely certain I don't hit a bridge.

There are bridge boards (painted boards partway in the water marked with the clearance at various river levels), but not for every bridge, and not always maintained.

The other controlling factor to consider is the Tenn-Tom, which is the usual route to the gulf, and which has a number of bridges at 52'.