A drainage system grate assembly for covering and allowing fluid passage into a watercourse includes: a crossbar having a plurality of spaced-apart upstanding knuckles defining a plurality of spaced-apart channels therebetween, each of the knuckles having a pin-receiving hole; a plurality of tread bars positioned in the channels between the knuckles of the crossbar, each tread bar having a pin-receiving hole axially aligned with the pin-receiving holes of the knuckles; and a pin positioned within pin-receiving holes in the knuckles and the tread bars, to secure the tread bars in the crossbar.