Water Resources Center
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
EVALUATION OF A POTENTIAL LAKE SITE
Examination of topographic maps is the first step in evaluating a potential lake site. Topographic maps can be used to determine the approximate size of the lake at a particular dam height, the drainage area for the lake, the gradient of the stream, the configuration of the valley, the steepness of slopes, and the accessibility of the area. Local improvements such as pipelines, high tension lines, radio towers, roads, cleared land, open land, major fence lines, houses, and other buildings are shown. Physical features such as streams, caves, springs, and sinkholes can also be observed on topographic maps.
After an evaluation of the topographic setting, a geologic and hydrologic evaluation of the proposed site should be made. The bedrock formations likely to be encountered can be determined from geologic maps, from well-log records, and from publications. The surface geologic evaluation can usually be limited to a walk-through investigation to observe geologic conditions that are not evident from maps or reports of previous studies. At the same time, observations can be made of the presence or absence of springs or seeps, whether the stream is losing or gaining, the type and thickness of soil, and the characteristics of bedrock (jointing, solution features, pinnacles). Subsurface conditions at the dam or upstream can often be inferred by surface conditions downstream from the dam.
Where adverse geologic conditions exist at a proposed dam site, they can often be avoided by selecting another site a short distance upstream or downstream, thus getting well above or well below the problem. Grouting the rock or soil with chemicals or cement, padding (an earthen blanket), and proper placement of the core of the dam can often be used to offset adverse geologic conditions.
When potential problems associated with leakage and structural (lake bottom) collapse are suspected, but are not in evidence at the surface, holes can be drilled on the center line of the dam to determine the subsurface conditions. Geophysical surveys using seismic and electrical resistivity methods may be used to complement the drilling program.
Exploration of the valley bottom to determine shallow subsurface conditions that cannot be prejudged from surface evidence (fig. 1 below) can often be done inexpensively with a tractor-mounted backhoe. Several holes can be excavated on the center line of the proposed dam to determine depth to bedrock, quality of material over bedrock, the presence of water, and estimate the cost of the core. Backhoe test pits can also be used to evaluate areas for borrow material for the earthen dam and core.
Figure 1 - Schematic diagram of a valley cross section showing uneven
Very few proposed lake sites will have all the geologic and hydrologic conditions necessary to insure a successful water impoundment or, conversely, have all the conditions that indicate high risk. A combination of good and bad features is usually the case. From the economic standpoint a decision is reached by weighing the positive factors against the negative. Several sources of technical information and assistance are available to help evaluate these factors.
DESIRABLE GEOLOGIC AND HYDROLOGIC CONDITIONS
There are five geologic and hydrologic indicators of a desirable water impoundment site (fig. 2): 1) adequate drainage area, 2) well-defined stream channel, 3) springs or seeps in the headwater area, 4) high groundwater level, and 5) high clay content soils with impermeable bedrock.
Adequate Drainage Area
One of the most important considerations in selection of a lake site is the contributing stream flow and drainage area. In addition to the initial filling of a lake, the water level must be maintained for extended periods without rainfall. Annual evaporation and rainfall can be nearly equal for a large part of Missouri. Thus, evaporation plus normal seepage makes it most important to have an adequate water supply.
Figure 2 - Successful lakes such as this, where recreation and housing developments
revolve around the lake, involve a high investment. Photo by Jerry D. Vineyard.
The desirable amount of drainage area or total water supply necessary to maintain a stable water level in a lake is difficult to determine. The runoff in a watershed is greatly affected by regional rainfall characteristics, topography, type and amount of vegetation, and soil and bedrock. Experience indicates that a minimum of 10 acres (4.0 ha) of drainage area to each acre (0.4 ha) of lake is desirable. As the drainage-area to lake-area ratio approaches 50:1, the storage capacity of the lake and spillway design become very important.
Drainage areas are easily computed on topographic maps, as shown in figure 3.
Figure 3 - Computing drainage areas on topographic maps. (Numbers 1, 2, 3, and 4 represent small lakes and their correspondiong drainage areas.) To draw the drainage area for lake no. 1, start at the dam at point (a), go up the valley wall to the right to the ridgetop at point (b), then go up the ridge line to point (c), follow the ridge to the left through points (d, e, and f), then drop down the valley wall to point (g), the other end of the dam. You have followed the ridge line around the lake and have outlined the drainage area. Follow the same procedure for lakes 2, 3 and 4.
On this scale map, a grid pattern of squares 5/16 inch (7.9 mm) on each side divides the area into 10-acre plots. Drainage area no. 1 has approximately 23 squares or 230 acres of drainage; no. 2, 110 acres; no. 3, 210 acres; no. 4, 220 acres.
Well Defined Stream Channel with Few Abnormalities
A valley that develops normally by erosional processes is relatively narrow, has a steep gradient in the headwaters and progressively widens as the gradient decreases downstream. Resistant layers of rock or water-loss zones prevent the normal eroding or downcutting of a valley bottom. Abrupt changes in the valley width or stream gradient reflect these abnormalities, which can adversely affect the success of a lake.
A continuous flow of water is usually indicative of a relatively impermeable valley (lake) bottom. As shown in figure 4 below, absence of vegetation in the channel, well sorted or stratified sands and gravels, clay-rich soil zones, and terraces are indicators of a gaining reach of the stream even though the stream may be dry at the time of investigation due to climatic conditions or small drainage area. Many other factors are considered in the evaluation of a stream channel, making the overall assessment one of observing many phenomena along the stream as well as within the watershed area.
Figure 4 -Schematic diagrams of valley cross sections showing a
defined channel (losing stream) and a well defined channel (gaining stream).
Springs or Seeps in the Headwater Area
Springs and seeps upstream from the lake site are related to bedrock permeability. The surrounding countryside is discharging water to the valley (discharge area) instead of the valley discharging its water to the surrounding countryside (recharge area). The probability of the valley (lake) holding water is greatly enhanced by these features. If the stream is perennial, springs may be contributing to this flow. The absence of springs may indicate that water in the uplands is seeping into the ground and escaping down valley as underflow.
High Groundwater Level
Springs, seeps, and perennial flow are evidence of a high water table level in many cases. A water table that is high enough to be intersected by a valley is likely to cause a perennial flow into the stream as shown in figure 5 below. Springs and seeps are ground water being discharged at the surface. Perennial flow caused by high water-table level does not always indicate impermeable bedrock in the valley bottom.
Figure 5 - Schematic diagram of a valley which intersects the groundwater level.
High Clay Content Soils With Impermeable Bedrock
Thick glacial till soils (Area 1), thick alluvial gumbo clays (Area 5 and flood plains of Area 2), and relatively impermeable shale bedrock (Areas 2 and 3) provide ideal lake sites, geologically. Residual clay soils (Area 2) vary considerably in their permeability, depending in part on their development history. A judgment of their relative water-holding capability must be carefully made and may require laboratory testing procedures.
Glacial till varies in consistency from sandy, silty clay to gravel and boulder clay. Most tills can be considered impermeable unless large pockets of sand or gravel are present.
Thick gumbo clays (Area 5) are impermeable and provide excellent material for lake construction where topography is suitable. The lateral persistence of the clay should be verified prior to construction, as permeable beds or pockets of sand can be associated with the clay.
The shale bedrock of Areas 2 and 3 makes excellent lake bottoms. The vertical and horizontal permeabilities of the shale are very low. Limestone bedrock, usually associated with the shale, is commonly fractured and can transmit water horizontally.