Hi All. Petra recently asked a question as to how sag ponds were formed. I thought about it for a bit then decided that in order for me to give her a answer I was going to have to do some searching. I found a answer although I’m sure it’s not the only one. Take Care…Don in creepy town

The measured vertical offset of scarps ranged from 0.2 to 18 meters throughout the study area. Between many of these scarp-like features and minor scarps, a down dropped bench could be found. Sag ponds commonly formed on top of these benches and when dry, they appear as local depressions.

Transverse ridges were found at many landslides in the study area. These ridges, paralleling the slope contours, located within the moving mass appeared as thrusts of displaced material. Many times a local depression, sometimes filled with water, was located directly behind these ridges.

Active Features
For planning purposes, any landslide that has undergone historic (about 100 years for the western U.S.) movement is considered active or as having the potential for renewed movement (McCalpin, 1974). This category also includes landslides which are currently moving. Landslides in this active category typically possess fresh, unmodified morphologic features. The head scarp is fresh, often jagged, steep (approaching 90°), unvegetated, and shows the original irregularities of the break. Closed depressions or sag ponds, often containing water are common on the down-dropped blocks. Pre-slide drainages are truncated and development of internal, marginal drainage channels have not begun. The slide toe may interfere with the modern valley, forming ponds upstream of the slide-toe dam (McCalpin, 1974).

1) sharp unvegetated, continuous scarps.

2) sharp unvegetated, shear zones, most with cracks extending outward and parallel to the main shear and some with slickensided surfaces,

3) sharp, open tension cracks above the main head scarp.

4) unmodified toes of debris, some with ponds upstream. Some toes entered the valley and became debris flows displaying marginal levees and

5) internal down-dropped blocks are rounded, but sag ponds and springs are still prevalent, with some of the sag ponds being dry or moist depressions.

Inactive-Young Features
Landslides that have not moved within historic time will have more subdued features than active landslides (McCalpin, 1974). Scarps become smoother (decrease in slope angle) due to erosion and begin to be vegetated. Internal depressions begin to fill with locally derived sediments and may appear as soggy areas. Marginal gullies begin to develop along the newly developed topographic boundaries. The slide toe is trimmed back from stream bank erosion, which may trigger smaller debris slides and slumps along the margin of the toe (McCalpin, 1974).

This best describes Petra’s study area.

Inactive-young features found in the study area were slightly harder to identify than active features. The inactive-young features found in the study area include the following:

1) slightly rounded, partly revegetated scarps,

2) shear zones partly revegetated, no visible fresh cracks extending outward and parallel to shear, shear zone becoming or is a gully,

3) internal vegetation is younger, different type, or different density than surrounding areas.

4) toes are revegetated and some have active landslides occurring on the oversteepened slope due to stream bank erosion, and

5) internal down-dropped blocks are rounded, but sag ponds and springs are still prevalent, with some of the sag ponds being dry or moist depressions.

• Sag Ponds