This is a transition from speculation about the size of streambed material to direct observation of it.

FORBES: Curiosity Finds Evidence Of Ancient Stream On Mars

Not to be outdone by its brother, Curiosity has found the first evidence of an ancient streambed on Mars near the Gale Crater.

FORBES: Curiosity Finds Evidence Of Ancient Stream On Mars

Mars rover Curiosity has found the remnants of an ancient streambed on Mars, providing still more evidence that water once flowed freely there.

FORBES: Curiosity Finds Evidence Of Ancient Stream On Mars

That provides pretty clear-cut evidence that this is an ancient streambed.

FORBES: Curiosity Finds Evidence Of Ancient Stream On Mars

Down the road from the park visitor center at Furnace Creek is Death Valley Elementary school, a flat-roofed building beside a dry streambed, which blends easily into the gray-brown landscape.

NPR: Students Blossom in the Desert

The new state land lies in Newcomb about a dozen miles below where the Hudson first spills out of Lake Henderson into a rocky streambed, about 300 miles before it flows into New York Harbor as a wide, flat waterway carrying barges.

WSJ: OUT THERE: Upper Hudson opening to canoeists