By Clint Calhoun
We are continuing our discussion on lake ecology for those of you who haven’t seen my two previous articles. Last time we began the discussion on lake ecology and the things that influence the lake ecosystem. This month we are going to talk a little bit about lake morphometry and why it has ecological importance.
Lake morphometry refers to the physical dimensions of the lake. A lake’s shape and dimensions can have a huge impact on its water quality and productivity levels. Dimensions that we are typically most interested in are the surface area (usually measured in acres), the maximum depth, mean depth, and shoreline length. This data is typically gathered through bathymetry or bottom mapping. Bathymetry can show how quickly the lake bottom slopes as well as reveal important physical features on the lake bottom such as sand bars, rock shelves, drop-offs and other features. In the case of Lake Lure, bathymetry can tell us the location of the sediment shelf on the main channel, features such as old stream bottoms, the location of our sewer line, and features such as structures (old bridges, trees, rock outcroppings, etc). Anglers are often very interested in bathymetric maps of lakes because of the information these maps are rich with.
Why are these dimensions important? Surface area is an important number because it helps with managing lake uses. For example, lake acres is a variable the Town uses as part of its boating model. A boat pulling a water skier will typically use more lake area than say a fishing boat or a non-motorized boat. Surface area is also important for calculating volume when trying to understand hydraulic residence time (the time it takes to completely renew a lake’s water volume), and even is taken into account as part of hydroelectric generation. Lake Lure, according to Geographic Information Systems (GIS) technology that the Town uses, is approximately 812 acres.
Depth is an important measurement because depth influences stratification and the proportion of water in which algae can grow. The Lake Lure Dam is 104 feet tall and also serves as the deepest part of the lake. Maximum depth behind the dam is about 93 feet. When we collect our water samples, we collect from the deeper areas of the lake so that we can understand how well mixing occurs in the lake and if internal phosphorus loading issues are occurring. In a lake, plant growth occurs in the area of water known as the photic zone. This zone is the area of lake having sufficient light to support photosynthesis. Lake Lure is a relatively deep lake so its photic zone is limited to the upper portion of the water column and along the shoreline. This limitation of available light reduces the ability of the lake to support large amounts of plant growth. Aquatic plants are necessary for a well-balanced aquatic ecosystem.
Volume is another important measurement because it tells you how much water is in the lake at any given time. Lake volume influences the dilution of inflowing nutrients and affects hydraulic residence time. This is important because if there is nutrient input and the lake has a long hydraulic residence time, then those nutrients stay in the water column much longer or settle out and bind to bottom sediments.
Another important measurement is fetch. Fetch is the longest uninterrupted distance over which the wind blows across the lake. Fetch is affected by the shape and orientation of a lake and varies based on wind direction and affects wave height and vertical mixing of water. The wind in Lake Lure typically comes predominately out of the southwest, but in winter time frequently comes from the north and northwest. Because of the lake’s shape (a cross) and orientation (arms are oriented in the four cardinal directions of a compass), wind gets funneled down the main channel and across the open part of the lake, resulting in some really big waves for a small lake. Those waves help to mix the water surface and the upper layer of the lake (the epilimnion).
Shoreline length is the total length of the shoreline encompassing the lake. It represents the total amount of lakefront available for development and for lake edge effects such as shoreline erosion, shoreline habitat, etc. Lake Lure has approximately 24 miles of shoreline (contrary to the 27 miles of shoreline that has often been published) based on GIS analysis. Those 24 miles of shoreline have been largely impacted by upland development through the construction of seawalls, houses, lake structures, etc. Very little of Lake Lure’s shoreline is dedicated as littoral habitat. The littoral zone is the shallow zone along the lake shore that is typically occupied by rooted aquatic plants. In highly productive lakes, the littoral zone supports a litany of biodiversity.
When bathymetric studies are performed on lakes, all of these dimensions are generated as part of the mapping process. Bathymetry is very useful because it paints a very important picture of what the lake physically looks like from a three-dimensional standpoint. Bathymetry data can then be used by lake managers to make informed decisions related to most anything that applies to a lake: shoreline development potential, ideal locations for fish habitat, dredging needs, infrastructural issues, invasive aquatic species management, etc. Lake Lure has never (at least to this writer’s knowledge) had a comprehensive bathymetric study. We have conducted small studies in certain areas of the lake, particularly where there were dredging needs or where the sewer line runs, but nothing on a large scale. The information that could be gained from such a study would be invaluable, and would go a long way towards helping us make informed decisions concerning the long-term health of the lake. Next time we’ll talk about lake stratification and mixing.
Clint Calhoun is a naturalist, biologist and Certified Lake Manager and has worked in Hickory Nut Gorge for over 20 years. He is currently the Environmental Management Officer for the Town of Lake Lure. Check out Clint’s blog at http://clintcalhounadventures.blogspot.com.