River Features: Holes Part 2, Anatomy & Types
· Lip/ Pour over
The upstream edge or hump of the feature, created by an underwater obstacle (rock).
Here is a boof of the pour over lip in Technical Difficulties, Upper Quartzville Creek, Oregon.
Here is the lip of a large mid stream hole in Last Chance, Salmon River, CA.
· Backwash/ foam pile
The aerated surface water moving upstream and feeding back into the reversal. Here is a hole that has it's own name, Harvey Wall Banger of Greenwall Rapid on the lower Illinois River, Oregon. Because the backwash is boxed in by a wall on the right and a rock on the left, it can be difficult to avoid being sucked back in.
· Boil line
The differentiating zone between the water that flushes downstream and the water that recirculates upstream. Here is a simplified version of where the boil line is on Double Dip, Quartzville Creek, OR. Generally considered unsafe to run due to the distance of the boil line from the lip and the lack of good safety options.
Here is a more realistic sketch line depicting the boil line on Double Dip, it generally matches up with the topography of the geological feature that creates the hole.
The water moving downstream out of the boil line. Harvey Wall Banger outwash pictured here.
Edges of the hole, either feeding upstream or downstream. Here are the smiley face hole corners: where the pocket meets some downstream current and flushes out the seam. Lawson Creek, a Tributary of the Illinois River, Oregon.
Types of holes
The corners are further downstream than the pocket, feeding out of the hole and into the downstream current. See above.
The corners are further upstream than the rest of the hole, feeding back into the hole. Pictured here is Canton Creek, a tributary of the North Umpqua, Oregon.
A drop of even height or even retentiveness all the way across.
A ledge hole, creating an even hydraulic even though the ledge is made by a boulder fence.
• Retentive / keeper. If the boil line is a long distance from the pour over, it is more likely that floaty objects will not escape, but cycle back into the hole repeatedly. The main drop at Greenwall Rapid, Illinois River, Oregon. The steepness, power, shape, and distance of the boil line indicate it is retentive. Most rafters go as far left as possible here.
• River wide
Unavoidable hole or holes across the entire channel. Pictured here is one of the Yellow Bottom Ledges of Lower Quartzville Creek, Oregon, about a 10 foot tall river wide drop.
• Backed up
A rock is in the outwash or boil line, stopping even the river bottom current from escaping. The same pic of Last Chance, with the rock backing up the hole circled.