| Jerry Weber is a professor of field geology at UCSC and has been a consulting hydrologist for 31 years February 10, 2004 Response to Jodi Frediani's Nescient Comments on Karst Terrains and the San Vicente Creek Sinkhole
By JERRY WEBER
Gentlemen and Ladies:
I never can figure out who is sending what to whom by looking at the top of an e-mail - all of the return path stuff. However, to whom it may concern, I would comment as follows.
A karst terrain is formed by very slow subsurface solution of limestone, limestone marble, and on some occasions other soluble rocks (gypsum, salt, etc) by water percolating through fractures and void spaces in the soluble rocks. Karst terrains form when certain geographic, geologic and hydrologic conditions are present. As one might imagine the solution of the soluble rocks occurs very slowly over very long time periods. We are talking about tens of thousands of years to hundreds of thousands of years. Karst terrains are also very common - more so in the eastern US, but are very uncommon in California. Santa Cruz County has two small areas of karst - one at UCSC (the core of the campus is built on karst) and the other in and around the limestone marble quarries at Davenport. Both areas exhibit a variety of hydrologic and geomorphic features common to karst - such as, sinkholes (dolines), sinking or disappearing streams, swallow holes, springs, and cave and cavern systems. Occasionally, sinkholes or dolines will appear without warning. I can presently think of 3 separate sinkhole forming events on the UCSC campus over the past 30 years. All were small, and were eventually sealed to prevent possible mishaps.
The active sinkhole in San Vicente Creek that I examined this past year is a wonderful example of how these features form. Over tens of thousands of years a void forms at depth (think cave or cavern) and eventually the overlying material begins to subside into the underground void. In the case of the San Vicente Creek sinkhole the subsidence was very slow - fortunately - otherwise the water supply for the town of Davenport would have been severed. San Vicente Creek simply disappears into a swallow hole at the upper edge of the collapsing area. This is typically what happens during the formation of a sinkhole in a stream bed.
I recommended that the trees be removed from the top of the subsiding area for several reasons. First, removal of the trees relieves some of the weight of the overlying material, thereby reducing the potential for rapid collapse, and possibly halting the collapse temporarily - perhaps for months to years. Second, it facilitated the movement of the Davenport water supply line to a more stable location (at least temporarily). It will also make it easier to move the pipeline permanently to another location, one that will not be threatened by the continuing collapse of the sinkhole. Third, it provided a greater level of safety to the persons working in the area during the moving and support the water line.
In an area undergoing subsidence it is not at all improbable for the ground to collapse under the weight of a person resulting in a fall of anywhere for a foot or so to many feet. Another factor of concern was the potential for trees to fall while people were working in the area. When I made the decision to recommend that trees be removed from the area of subsidence numerous trees were already severely tilted or had fallen. It is important to point out that the potential for sudden collapse in the area remains. Consequently, any future work in the area should take extra precautions while working on the area of subsidence - small areas could collapse suddenly, without warning.
In regard to the concerns regarding the relationship between timber harvesting and sinkhole formation: there is no relationship between timber harvesting at any time in the past and the formation of the sinkhole [emphasis added]. This would presume that cutting a tree and removing mass from the top of the area of subsidence in some fashion would accelerate the process of solution of calcium carbonate to the point that a cavern system and sinkhole forms where there was none before. As pointed out earlier, this is process that operates on time scales of thousands to tens of thousands of years (in some areas hundreds of thousands of years); and removing trees removes part of the driving force that creates the collapse. To put it simply, the notion that the timber harvesting could have created the collapse is absurd [emphasis added].
In regard to fish, sediment, erosion, stream profiles, etc., I would comment as follows: It is highly probable that San Vicente Creek over the past 10,000 to 80,000 years has had similar events occur within the stream. The entire limestone marble block that lies within the drainage is laced with a variety of caverns, caves, narrow passages and enlarged joints and fractures. This means that over time the stream will simply disappear - get swallowed up and go underground for a distance and then surface again as a spring. It may do this intermittently - on a seasonal basis or on a time scale of many years. A similar condition exists today in the upper portion of the Wilder Creek and Cave Gulch drainages just west of Santa Cruz. This may not be beneficial to fish, but it is simply part of the natural system that occurs in karst terrains. In regard to the total amount of water in the system...unless there is a process that I'm not aware of...it would be the same. It certainly isn't going underground into some huge void and disappearing forever from the face of the earth. Since the limestone may be replenishing groundwater in sands that lie west of the crystalline basement complex, some of the water could be moving through the subsurface into that area, but that is already an ongoing part of the groundwater system in the region. So there is no reason to expect a net loss of water in either the groundwater or surface system as a result of the formation of this, or any other, sinkhole.
Sediment is common in cave systems. When a stream enters a swallow hole, its sediment goes with it. Therefore, there will be sediment in caves, lots of it. Some of the sediment obviously will pass through and continue downstream. When a sinkhole collapse is in progress, in most instances, there will be more sediment available to the stream. So, there will be more sediment moving through the system both above and below ground. The effect of the cutting the trees on the area of subsidence will not add any sediment to the stream that will not end up there as the collapse continues. As mentioned earlier, it is possible that the cutting of the trees will slow down and/or postpone the total collapse of the area, which would actually result in a decrease in the sediment load in the immediate future. In the long haul the sediment load will be about the same.
I have not been out to the area for several months, so I don't know what the present condition is at the site. We can all rest assured, however, that the subsidence will continue; at what pace nobody knows. If possible I would be interested in returning to the site sometime in the latter half of the month when my work load is reduced. This is a relatively rare event, particularly in California, and I would like to continue to observe the process.
Please feel free to distribute these comments regarding the general nature of karst terrains and the potential effects of logging as you see fit. I think it is important to educate everyone involved in the nature of the process.
Jerry Weber
Jerry Weber is a professor of field geology at UCSC and has been a consulting hydrologist for 31 years.
Link to the email by Jodi Frediani that prompted Jerry Weber's response | 
