Lake Shape
Lakes can vary in three dimensions: length, width, and depth. The shape and volume of a lake, referred to as lake morphology, often relates to a lake’s age, origins, chemical makeup, and the organisms that live within it. Lake shape is influenced by the surrounding landscape and controls much of what goes on physically and biologically underwater. For example, when lakes have many small inlets and bays, these areas can warm up quickly and are not affected by winds that can pick up speed over the large open area of a typical lake body.
Shape Characteristics
Figure 1: Lake shape affects the temperature and wave conditions on a lake. The fetch, or the maximum distance an airmass travels across a lake, can be measured along the length (L) of a lake. Width (W) is measured as the maximum distance at a right angle to L. The fetch often determines the maximum size of waves and how deep a lake mixes.
The length of a lake is measured as the longest straight-line shoreline-to-shoreline distance across a lake. Often, the length is an important factor regulating the local weather climate. Winds can pick up speed over open water, which is much smoother than the surrounding landscape. The length of a lake is also called the fetch, which is a term that describes the longest distance an air mass can travel across a lake (Figure 1). In most lakes, the fetch determines the size of waves and how deeply warm surface waters are mixed. Wind travelling over a long fetch is able to cause significant mixing in a shallow lake1. Its estimated that a fetch greater than about 60 miles (100 km) is required to generate lake-effect snow2.
Another important measurement is lake width (“W” in Figure 1), taken as the longest shoreline-to-shoreline measurement at right angles to the length. Common lake shape measurements other than length (fetch) and width include the maximum depth, the average depth, and the relative depth, which is a ratio of the maximum depth the to the lake’s diameter. Typically, the maximum depth of the lake can be found at roughly central location or at some distance from the shoreline. Most lakes have an average depth of about 10 meters. Depth can frequently predict the productivity of the lake, or how much photosynthesis it fosters, since a shallow lake will have greater exposure to both sunlight and nutrients3.
Shoreline
Figure 2: The shoreline characteristics of lakes can vary dramatically. Here, the shape differences also highlight differences in the origins of the lakes. Compare the jagged shoreline of the reservoir on the left with the smooth shoreline of the natural lake on the right. Reservoirs can have a jagged shoreline because they often consist of flooded river valleys. (Modified from 5.)
The shape of a lake’s shoreline is strongly connected to lake biology. The characteristics of the shoreline — whether it is jagged or smooth, curved or angular — affect what happens underwater. The more jagged and indented the shoreline, the more the lake is affected by inputs from the surrounding land through runoff or groundwater leaching3. Many organisms depend on natural, unaltered lake shorelines for shelter and nesting sites. Often, home owners who live along lakes remove debris and vegetation to clean up the lake shoreline, but doing so can hurt underwater organisms, including larval fish and amphibians. Trees and branches that fall into the water, referred to as coarse woody debris, are important shelters and nest sites for many creatures, including fish and their invertebrate prey. Also, as wood and plant debris decompose, they release many important dissolved nutrients that will feed plants and microorganisms4.
Most lakes are roughly circular, but some lakes can be highly irregular in shape. Compare these two lakes in Figure 2.
Littoral Zones and Pelagic Zones
Figure 3: The littoral zone of lakes encompasses the shallow regions where aquatic vegetation can attach and grow. The pelagic zone encompasses the deeper areas of lakes where phytoplankton dominate. Modified from 6.
Lake scientists classify the areas of an individual lake based on gradients of depth that increase away from the shoreline. The shallow perimeter of the lake is typically referred to as littoral. Littoral zones are shallow enough that sunlight is able to penetrate to the sediments and support photosynthetic plants. Areas where attached aquatic vegetation (called macrophytes) can be seen emerging from the water are typically littoral. The depth of the littoral zone can depend on the transparency of the water, where vegetation can grow in deeper areas when a lake is more transparent.
The open water of the lake is referred to as the pelagic zone. Here, photosynthesis and primary production is dominated by phytoplankton which can remain suspended in the water column3.
Want to experience what it feels like to fly through a lake underwater? Scientists and lake managers often measure the underlying shape of a lake — termed bathymetry — with sophisticated sonar equipment. Lake Tahoe, the second deepest lake in the U.S., has detailed movies highlighting the lake’s bathymetry7.
Sources:
- Hall, S.R & Rudstam, L.G. 1999. Habitat use and recruitment: a comparison of long-term recruitment patterns among fish species in a shallow eutrophic lake, Oneida Lake, NY, U.S.A. Hydrobiologia 408/409,101–113.
- Wikipedia. Lake Effect Snow. 2010. Retrieved online at: http://en.wikipedia.org/wiki/Lake-effect_snow#Fetch
- Wetzel, Robert G. 2001. Limnology, 3rd ed. San Diego: Elsevier.
- Christensen, D. L., et al. 1996. Impacts of lakeshore residential development on coarse woody debris in north temperate lakes. Ecological Applications, 6 (4), 1143-1149.
- The Water Line. Reservoirs vs. Lakes. 2003. Retrieved online at: http://www.lmvp.org/Waterline/winter2003/dam.htm
- New Zealand Ministry for Water and the Environment. 2010. Draft Guidelines for the Selection of Methods to Determine Ecological Flows and Water Levels.
- U.S. Geological Survey. 2010. Lake Tahoe Data Clearinghouse. Retrieved online at: http://tahoe.usgs.gov/movies.html
