He "became quiet and started to shake" when asked if he had recently been to New York, according to the criminal complaint filed in Pennsylvania.
Being smart can lead to arrogance, which leads to stumbles. like carrying evidence, dining in public, etc.
Bodies of water are popular for disposing of things, because it's a huge PITA to find things there. Imagine how hard it might be in a large local pond, and then multiply that in complexity for a rather big river. Or an inland sea like the Great Lakes. Don't even multiply for those; it's a stupidly big irrelevant number.
He totally did. Find a trash can, and put it in (though maybe disassemble the gun first, and dispose of it in pieces in multiple locations).