The water is that color mostly due to Tannins in the water. Much of the water that enters the river flows thru hardwood forests with lots of leaf litter and thru peaty bogs. But the river does have a big history of pollution. It was at the center of the textile industry in the NE. Lots of water powered mills along it and after dying the fabric they would dump it straight into the river so the rivers color would change frequently.... I have heard stories that before the clean water act you could almost walk across it on all the crap that was in the river. It has been used as a dump since we first started settling this country. It is a big river, about 4x the volume of water in the merrimack at lowell than the colorado in glenwood. But much wider and deeper and it has a much lower gradient and current speed. A great place to disappear any problem you might have. Cats are not the only things that get thrown in bags with rocks and dumped in the river around there.
"I have come to kick ass and chew bubble gum, and I am all out of bubblegum" Rowdy Rodney Piper in "They Live"