"You look like you've seen something awful."
The River ... an attempt at explaining it, atleast, because MAN its a lot...
Listen, Gato is so purposefully obtuse about this shit and *I* get it because I've consumed so much content but it feels hard to explain so I'll try the best I can and quote her.
'The river' is a plane of existence/non-existence that's portrayed as an endless expanse of water in the area between life and death; Gato describes it as 'a place for the dying'.
"It’s generally inescapable. Despite it having no shores and endless expanse of ‘water’, it’s called ‘The River’ because it’s flowing in one direction. Depending on one’s circumstances, they may find themselves in The River where it’s shallow and the pull isn’t very strong. There is nowhere to go however, and the longer you stay, the more you forget of your life.
There is a ‘tiredness’ that compels people to lie down and let the flow carry them.
Lawrence entered The River as a child when he drowned at the bottom of a lake. He had a curiousness and love for The River and he wandered, refusing to lie down. He found a way back, which heavily violates the laws of nature."

The river is mentioned multiple times in Lawrence's route, and whether or not the MC has knowledge of the river affects an ending...

Lawrence - Dude Is Fucking Decaying
With each visit to the river, Lawrence becomes less and less human; essentially decaying and rotting away. The longer Lawrence spends in The River, the more his true shape changes...
Animals lifespans tend to shorten around him, food spoils faster, and he gives off a sortof 'I'm dead.' smell. To quote from Gato's blog, "Insects [ and any other carrion feeder ] are attracted to him, but they die when they get too close or if they stay near for too long. For them, it’s like if radiation smelled good."
The MC comments on a sickly smell; and upon realizing it's coming from Lawrence, notes it's the scent of something rotting.
Gato has stressed that in BTD2, he's not immortal, he's death impaired. In the 100% post-game scene with Ren, it is revealed that Lawrence's body is missing from the closet; as if he just got up and left. Gato confirmed that's exacly what happened.
This reveals to us that Lawrence doesn't *really* die in any ending.
Man, this paragraph has mostly been 'she said' and 'she confirmed', but I do wanna bring up one more fun tidbit Gato said; following the 'Lawrence Kept You' ending, if the MC were to die, Lawrence would physically pull you out from the river himself. Essentially can't escape him and shit...