Each is different.  Even the upstream canyons–say, Hansen, Forgotten, Annie, Halls–each would be a National Park anywhere else were it well known.  They are superb.  But the lower saw-cut canyons are each precious gems in the desert.  Gigantic gems.

You tie up your boat on the river bank and set out toward the opening in the wall, perhaps a slit in the massive sandstone many hundreds of feet high.  Sometimes you walk on a dried mud flat with thousands of tiny toads scurrying about and chinmeying in and out of the deep cracks in the mud.  Sometimes you wallow or “swim” in greasy-looking black mud.  If so, there is almost always a clean, cool (cold, in winter, maybe covered with ice) pond in which to wash off the mud.  Then you enter a natural cathedral, designed by an architect no human could match.  Vaulted walls, overhanging walls, fluted walls, patterned walls, intricately carved walls.  Winding corridors, branching corridors, echoing corridors, secret corridors, corridors illuminated by a shaft of sunlight making the walls glow with a fire-like hue, corridors illuminated by only the blue sky spreading a ubiquitous blue glow, dark corridors, corridors so cave-like you must carry a flashlight in the daytime.  Gargantuan hallways, narrow, twisting, passageways, rock-roofed rock amphitheaters, small private carrels.  Perhaps a floor flowing with a trickle of water, taking plunges here and there (and sometimes challenging your climbing skills).  Perhaps a pond asking you to demonstrate your swimming skills.  But most are easy walking, any challenging rubble having been swept out by the occasional flash flood (and that, too, is something you must adequately analyze as a thinking human explorer).  The great stream alcoves are awesome in a sense that renders that description useless elsewhere.  Perhaps a hundred of them total, virtually all near the mouths of the side canyons, near the Colorado River.  One canyon wall leans over the opposite wall in response to the cutting momentum of water and rock washing downstream in a sharp curve, washing and grinding in a canyon of rock extraordinarily subject to erosion by water and debris flow, but sufficiently strong to remain as a leaning canyon wall after the deluges.  This is Navajo sandstone.  Slightly soluble in water, but the water is that of the desert of the Colorado Plateau: scarce.  Unique rock in a unique place.