Water Resources Center
Continuation of Excerpts from:
A Guide for the Geologic and Hydrologic Evaluation of Small Lake Sites in Missouri
(Water Resources Report Number 31)
by Thomas J. Dean, James H. Barks and James H. Williams
available through the Center's Publication Desk at
573-368-2175 or download online
UNDESIRABLE GEOLOGIC AND HYDROLOGIC CONDITIONS
There are many geologic and hydrologic conditions that are indicators of undesirable water impoundment sites. Most of these are related to: 1) dry, poorly defined stream channel, 2) low groundwater level, 3) karst topography, 4) pinnacled bedrock, 5) severe jointing, 6) springs downstream from the dam site, and 7) coarse textured material in the valley bottom and slopes (fig. 6).
This obviously unsuccessful lake should never have been constructed in this location because of adverse geologic conditions. It would be uneconomical to repair. Photo by David Hoffman.
Dry, Poorly Defined Stream Channel (losing stream)
Generally, a dry, poorly defined stream channel, as shown in figure 4, is an indicator of a losing stream. Vegetation growing in the stream channel; unsorted alluvial material with boulders and gravel; and a very small channel in relation to the size of the drainage area are all characteristics of a losing stream. Absence of terraces, angular topographic profiles, and a host of other surface features aid in the identification of losing streams. When water from rainfall enters the valley, it seeps underground into thick gravels or into bedrock openings. In the absence of surface flow the gravels and sand (alluvium) are not carried away and little sorting of the alluvium takes place. Vegetation grows in the channel and the streambed is not always clearly defined.
Investigation downstream may reveal how deep the lost water goes before it starts moving horizontally. If the water reappears a short distance downstream from the proposed dam, the water-loss zone may be shallow and can be intercepted by the core of the dam. If no reappearance is noted downstream, the lost water might be going very deep or possibly leaving the valley under the valley walls. In this case, the lake site would be considered unfavorable for water impoundment.
Low Groundwater Levels
A low groundwater level has many causes, but generally it reflects conditions that are undesirable for lake sites. For example, a site underlain by cavernous bedrock with a low groundwater level would make a poor setting for a lake.
Information on groundwater levels can be obtained from nearby water wells or well logs. If wells indicate a low groundwater level and the valley and its stream have not developed normally, the basin is probably very permeable and would not hold water.
To the left, the schematic diagram showing karst features (including sinkholes) and solution work in joints and bedding planes. The topographic map below shows karst features.
Karst Topography (Sinkholes and Caves)
Karst topography is characterized by the presence of numerous sinkholes and caves which indicate very high permeabilities in the soil and underlying bedrock. As shown in figure 7, a sinkhole is simply an area where the roof of a cave has collapsed. Water moving in the cave carries away the fallen debris and the sink enlarges itself until it is stable, usually resulting in an inverted cone-shaped depression. The depth of sinkholes varies considerably from place to place, but in Missouri a depth of from 70 to 80 feet (21 to 24 m) is not uncommon. This depth may well put the cave at or below the valley bottom.
The orientation of the cave and its relationship to the valley cannot always be determined. The cave may parallel the valley, be at right angles to it, be above the valley bottom, or well below it. Caves are natural outlets for water, so where sinkholes exist at or near a proposed lake, the site is considered geologically unsuitable.
Pinnacles (spire-shaped pillars of rock) are formed by solution along joints or cracks in the bedrock, or by differential weathering caused by abnormalities present in the rock. This phenomenon is not too unlike the sinkhole-cave relationship. The space between the pinnacles is not expressed on the surface as a depression, but rather as a filled depression (fig. 16). Thus, there may be no surface evidence of the problem. Bedrock may be present at the surface at one point, but 20 to 30 feet (6 to 9 m) away the soil may be many feet thick. The thick soil filling the crevice between the pinnacles is usually very permeable and may result in serious water loss from the lake (fig. 8).
All bedrock is fractured or jointed to some degree, but most joints are very small and cannot be seen except on close examination. These cracks are generally of little concern. Large joints, however, can cut entirely through a hill or ridge and become natural outlets for water. Slumping of large blocks of rock downhill may cause the joints to open up to crevices of several inches to several feet in width. Severe jointing, like pinnacled bedrock, is not readily noticeable from the land surface, but can usually be suspected from joint development in nearby bedrock exposures. Where soil cover is uniform, however, the problem may be overlooked unless subsurface exploration is conducted.
Figure 8 - Solution work caused by running water makes for a
permeable lake bottom. Photo by Jerry D. Vineyard.
Springs Downstream from the Dam Site
Springs downstream from the dam site are indicators of a geologic condition that has contributed to the failure of numerous lake projects. The spring is a natural outlet discharging water collected from several small underground feeder systems. Hence, the chance of impounded water in the lake finding its way into one of these systems and from there into the main channel of the spring is very great.
There are generally two types of downstream springs. One is the alluvial spring in which water is lost from the stream into the alluvium upstream, flows at shallow depths through the alluvium and reappears downstream. The other is the bedrock or bedding-plane spring in which water moves vertically into the alluvium and bedrock, then horizontally through the bedrock and emerges at an outlet down-stream. In an alluvial spring the water can usually be intercepted with a properly designed core under the dam. However, water moves at much greater depths in a bedrock spring system and is often difficult or impossible to intercept. Therefore, the dam should be placed well below or downstream from a bedrock spring outlet.
Coarse Textured Material in the Valley Bottom and Slopes
Most valley slopes in bedrock areas have a natural clayey soil cover if the slopes are gentle enough so that erosion does not remove the soil as fast as it forms. In the valley of a losing stream the floodplain normally has a coarse textured soil composed of silts, sands, and gravels overlying bedrock. If both the slope soils and alluvial soils are coarse grained sands and gravels, leakage problems are common. The sorting of slope and alluvial soils assists in determining the classification (losing or gaining) of the stream, as mentioned previously.
Glacial soils (Area 1) have pockets or lenses of sand and gravel, as shown in figure 9 below, which cannot always be predicted from surface evidence. These coarse textured materials create serious leakage problems where they are located in the valley slopes or lake bottoms.
The best assurance against water loss from a lake, whether it is rapid flow or slow seepage that affects the waterline in dry weather, is a fine grained, plastic clayey soil 4 to 5 feet (1 to 2 m) in thickness covering the bedrock or sand and gravel deposits on the valley slopes and bottom. This pad of material would help prevent water from entering the permeable bedrock to be transmitted around or under the dam, or through the hill to the next valley.
Figure 9 - Schematic diagram of a valley cross section with sand
pockets in glacial till and deep alluvium.
Additional chapters include:
Typical Lake Settings, Construction Considerations, Physical Factors That Affect Construction Costs, and Repairing Leaking Impoundments.
To Download or Purchase This Book
This book and other geological or hydrological books and maps may be purchased from the Water Resources Center's publication desk, located at 111 Fairgrounds Road, Rolla, Missouri 65401 (for directions); or by calling 573-368-2175; or use the department's toll-free number (800) 361-4827; or download online.