Cuyahoga River Fire Worse

That was my first thought. A chemical factory/warehouse located adjacent to the bridge, or a bit down stream. Adjacent is a old district of commercial buildings and row houses all jammed together with no mid 20th Century fire lanes & set backs...

But a cargo ship creating a Halifax or Texas City size explosion would be pretty spectacular.
 
Well, it's unlikely you'll have something like Texas City at the Cuyahoga Docks (IMO) due to what's normally being carried in there, ie, bulk cargoes such as coal, coke, ore, etc etc, large quantities of fertilizer isn't something that would show up there by ship. That stuff came in via rail. However, having the piers burn out, or the fire spreading (IIRC there are some petroleum tanks, or were, along the river), that could do it. If the fire started onshore with an industrial explosion that spreads to the river, then you have a major conflagration. IIRC Sherwin Williams or another of the big paint companies are along the river shoreline. Depending on how big the fire gets and how fast it spreads, it might take out the railway bridge down in the flats. You could actually have quite a few people killed, if the fire takes place down in the flats or just upstream as access/escape routes are limited. Big fire, no way out, river burning, catastrophe.
Having lives lost would give Nixon a fine platform to launch the EPA, and I could see it being stronger and more environmental regulations being passed sooner. All in all this is pretty plausible IMO.

OSHA gets created just as in OTL, along with the EPA. I see them being larger and more powerful though. The commercials we saw back then of the crying Indian, lots more of that type of thing. It may well cause a deeper commitment to environmental causes in the US, in fact I am certain it will. One casualty of this would be offshore drilling, and couple this with the gas crisis that comes a bit later, we will see more of a push for smaller, more fuel efficient vehicles and a commitment to mass transit.
 
maybe the fire ignites the coal dust from ships loading up

Coal dust on the ground isn't a real problem. Dust in the air would be, but its difficult to ignite unless your in a closely enclosed area (cargo hold, coal bin, factory floor etc etc) BUT, you could ignite the piles of it that will be about. Even so, that will produce more of a smoldering, hot mini mountain of flaming coal that they'll have to eventually put out. The real damage in a huge fire in the "Flats" area will be to people, buildings, bridges wharves and ships. As Carl postulated, a bigger fire on the river gets out of hand. I don't think it will spread out of the "flats" area, as it would be easy to quarantine it there along the river, although they may need to dynamite some buildings at the top of the slope to make a firebreak.
 
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