[onedoug]

I know Mars is extremely dry, yet given the frequent planet wide dust storms thereon, I’d expect considerably more erosion and burial from between 1.8 and 3.5 billion years.

[bigheadfred]

I’d expect considerably more erosion and burial from between 1.8 and 3.5 billion years.

Right. The surrounding terrain is pocked with craters. I see none in the riverbed.

[The Cajun]

Yup, you would almost expect to see the same texture of the surrounded landscape at the bottom of the river after a billion years or so due to fill, especially since it's very deep.
Kind of a natural trap for dust.