Determining which 20 canyons to include in the 20-20 Challenge was by far the hardest part of this project. We finally settled on a 3-step decision process:
 

Step 1	Distinguish "classic" canyons from valleys, side canyons, and other landscape features.
Step 2	Eliminate canyons lacking accessible trails and lacking challenge.
Step 3	Choose the 20 deepest qualifiers.

Step 1 - Definition of a "Classic Canyon"

First, here are some definitions of canyons from various sources:

"A deep gorge or ravine at the bottom of which a river or stream flows between high and often vertical sides." -Oxford English Dictionary

"A deep valley with high steep slopes. Canyons are characteristic of regions where, owing to the aridity or to great slope, the downward cutting of the stream greatly exceeds weathering." -Websters

"Valleys which are narrow and deep, but small are often called gorges. Similar valleys of larger size are called canyons. The sides of gorges and young canyons are sometimes nearly vertical, but the sides of the larger canyons are rarely so. The distinction between a canyon and a valley which is not a canyon is not a very sharp one, and in regions where canyons abound, the term is often applied to all valleys." Salisbury (1923), p. 105.

"A steep-walled valley or gorge in a plateau or mountainous area. The highprecipitous slopes impress the observer more than the flat land which may occur along the stream, and this impression depends on the distinction between canyons and other valleys. Rice (1941), p. 62.

"Submarine canyons…steep-sided…of a winding character and have all the characteristics of valleys draining a high plateau." Hills (1947), p. 61.

(These definitions are taken from Stamp, Dudley L. (ed.). 1961. A Glossary of Geographical Terms. New York: John Wiley and Sons.)

We based our canyon criteria on these definitions, our own opinions, and field trips with students. Because our purpose is recreational, we developed a more restrictive definition of "classic" canyons that provide a hiking experience that is more unique to the southwest. So while the criteria are primarily geomorphologic, they also include recreational factors.

1. Steep walls on both sides parallel to the steam course. Walls at some point in the canyon should be 45º or steeper on both sides.
2. Above the walls there should be a relatively flat rim of fairly consistent height. The canyon should be cut down into a plateau rather winding through mountain, and looking up, the hiker should be aware that there is a distinct rim. We determine this by searching for an abrupt and consistent change of slope from steep to flat at the top of the canyon.
3. The bottom should be relatively flat, under 10º slope on average. In Arizona, many large ravines running down the sides of mountains are referred to as canyons (see Salisbury's definition), but most out-of-state visitors would not consider it a canyon if hiking along the steam bottom takes one to a mountain summit.
4. Walls may be broken occasionally by side canyons, but not if there is little wall between side canyons, and not if the side wall is broken into free-standing rock formations.
5. This last criterion is used to determine whether a side canyon is an independent, stand-alone geographic feature, or an indentation along the side of another feature. We decided that for a hiker to feel they are in a separate canyon, the length:width ratio must be greater than 2:1, where length is measured from canyon entrance to end of canyon (not the end of the trail) following the canyon centerline, and width is measured at canyon entrance from rim point to rim point. This criterion is also applied to canyons eroded into escarpments, such as the Mogollon Rim.

For safety reasons and for environmental reasons, the 20/20 Challenge is restricted to hikes with trails.

1. Canyon hikes may have a trail to the bottom or along the bottom.
2. Canyon hikes must be a minimum of 5 miles round-trip or an elevation gain/loss of at least 1,000 feet.
3. Hikers should not need a heavy-duty, 4-wheel drive vehicle to reach the trailhead.
4. If a permit is required, it must be low cost and easy to arrange.
5. If there are multiple trails, we chose the one recommended by Forest Rangers or the one that is the most challenging.

We measure depth from rim to floor at the deepest point of the highest side of the canyon. It worked out rather nicely, with exactly 1,000 feet being the cut-off point for depth. The next canyon on the list (#21) would have been Aravaipa, which is only 840 feet deep. Too bad it didn't work out this well for the peaks, for which we had to dip below the 9,000 foot mark to get 20 with trails.

Discussion of Special Cases and Near Misses

Even with these concrete criteria, there were still many tricky cases that could have gone either way. Here's how we decided the marginal cases.
 

In	Out
Salome Creek (north of Roosevelt Lake). The land surrounding Salome is not exactly a flat plateau, but there is a rather consistent slope change that defines a rim. Also, although there is no official trail into the deep part of the canyon, there is one from the road to the steambed. The Tonto Ranger District approves of hiking further into the trailless canyon, and in fact there are informal trails, and it is impossible to get lost.	Cibecue Canyon (Salt River Canyon area). Spectacular Cibecue Canyon is on the San Carlos Indian Reservation. There is a $10 fee per person to hike the first half-mile up to the falls. Beyond that, hikers must hire guides whose costs range from $95 to $300 per hiker, depending on the group size. Because of lawsuits, the canyon may be closed completely in the near future.
KP Creek (Blue Range). The land surrounding KP is not a perfectly flat plateau, and in fact the trail ascends up the side of the canyon to Blue Peak. But the rim is fairly distinct and consistent, and the canyon is quite deep. Walls are just steep enough, but forested.	Rogers Canyon (Superstition Wilderness). Rogers is deep enough and has parallel steep walls, but there really is no distinct rim. The rim is more like an undulating ridge line.
Bear Wallow (eastern Mogollon Rim). Like KP Creek, this is another high-elevation forested canyon.	Boynton and Fay Canyons (Sedona area). Trails are too short.
Tuckup Canyon (North Rim). Length:width ratio is exactly 2:1.	Sterling and Casner Canyons (Sedona area). Length to width ratio do not satisfy 2:1 criteria.
Hack and Jumpup Canyons (North Rim). Despite being side canyons of the Kanab Creek, itself a side canyon of the Grand Canyon, these two canyons pass the length:width test with 6:1 and 2.7:1 ratios.	Munds Canyon (Sedona area). Thompson Point trail does not go into Munds Canyon per se, but rather is right on the corner of the canyon entrance.
Havasu Canyon (west of South Rim). On the Havasupai Indian Reservation, hiking permits are easy to reserve and the fees are low.	Mooney Canyon (Sedona area). No solid wall on east side.
	Loy Canyon (Sedona area). No distinct rim at top of walls. Must go far up side canyons to hit the rim.
	Muav, Tanner, Nankoweap, Grandview, Hance, North, and South Canyons (Grand Canyon area). None of these side canyons pass the length:width test of 2:1. Generally, hikers would feel like they were in the Grand Canyon.
	Tonto/Haigler Creeks (Payson Area). Broken topography is too mountainous to afford a true rim, especially where the trail crosses the creek. Devil's Gate feature is too short in length to qualify as a canyon.
	Pine and See Canyons (Mogollon Rim Payson area). Pine Canyon is a large eroded indentation into the Mogollon Rim, but its length:width ratio of 1.4:1 is less than 2:1. See Canyon has an even lower ratio.
	Canyon de Chelly (Four Corners area). Although it is a National Monument, spectacular and fascinating Canyon de Chelly is still farmed by the Navajo tribe, and has many sacred sites. The only trail that is open for unguided hiking is the White House Ruin trail, which is not deep enough to qualify for the Challenge.
	Burro Canyon (Prescott area). Not deep enough.
	Aravaipa Canyon (northeast of Tucson). Not deep enough, but close (840 feet). Wilderness area with perennial stream known for incredible bird-watching.
	Pima Canyon (Tucson area). Despite sheer walls, this is a glorified ravine up a mountainsides. If you follow the streambed of Pima Canyon, you climb over 3,000 feet toward the top of Mount Lemmon.