Study Notes on Lakes and Reservoirs: Physiography

Formation: Geological Processes

• Definition of a Lake: A lake is defined as a very slowly flowing or nonflowing (lentic) open body of water in a depression and not in contact with the ocean.

  • Saline Lakes: The definition includes saline lakes but excludes estuaries and other mainly marine embayments.

  • Comparison with Wetlands: The distinction between small shallow lakes or ponds and wetlands is not clear.

  • Intermittent Lakes: These are those that dry sometimes; they are distributed sparsely throughout the world, with greater numbers in arid areas.

• Geological Influences on Lake Formation:

  • Areas with more precipitation and suitable geological conditions lead to the formation of permanent lakes.

  • Geological History Influences: For example, northern North America has relatively more lakes than wetlands.

  • Human activities have significantly altered the distribution of lentic habitats, creating many ponds and reservoirs.

• Lakes and Small Bodies of Water: Lakes are often larger than small ponds and play vital roles in the ecology by supporting various life forms and chemical processes.

  • Small lakes have biological and physical characteristics distinct from larger lakes.

  • The geological processes that lead to the formation of lakes include tectonic movements, glacial activity, and other natural events.

• Lake Formation by Geological Processes:

  • Tectonic Movements: These processes form some of the largest and oldest lakes, such as the Great Rift Valley which formed Lakes Edward, Albert, Tanganyika, Victoria, Nyasa, and Rudolf.

  • Graben Lakes: Formed where faults allow a block to slip down. Example: Lake Baikal.

  • Horst Lakes: Formed by tilted blocks leaving a depression.

  • Damming by Natural Processes: Landslides, lava flow, glacial moraines, and beaver activity contribute to the formation of lakes.

• Types of Lakes and Their Formation Characteristics:

  • Table 7.3 summarizes different types of lakes and their formation processes:

  • Tectonic Lakes: Formed by earth movement; can be very deep and old (e.g., Lake Baikal).

  • Pothole or Kettle Lakes: Formed by glacial activity; often small.

  • Moraine Lakes: Created by debris dams created by glaciers.

  • Earthslide Lakes: Formed when landslides dam a stream.

  • Volcanic Lakes: Created by volcanic activity (e.g., Crater Lake).

  • Dissolution Lakes: Resulting from limestone dissolution.

  • Oxbow Lakes: Formed when a river bend is cut off.

Lake Habitats and Morphometry

• Subhabitats of a Lake:

  • Lakes are divided into different zones:

  • Pelagic Zone: Open water above the sediments.

  • Profundal Zone: Benthic habitat below pelagic waters, typically with fine silt or mud.

  • Littoral Zone: Shallow areas where light reaches the bottom, allowing photosynthesis.

• Morphometry:

  • Morphometry is the study of the shape and size of lakes and their watersheds.

  • Key measurements include area (A), maximum depth (z_max), mean depth (z), and volume (v).

  • Volume is calculated by the equation: $v = A z$.

  • Shallow mean depth typically indicates higher productivity due to factors like wind mixing of nutrients.

• Water Retention Time:

  • Determined by the average retention time (R) calculated as follows:
    $R = \frac{v}{L}$, where L is the loss of water due to evaporation or outflow.

  • Retention time varies from hours in small ponds to thousands of years in large lakes.

• Shoreline Development Index (DL):

  • Measures the degree of convolution of a lake's shoreline compared to that of a perfect circle.

  • Calculated as:
    $D<em>L = \frac{L}{2\pi A</em>0}$, where L is the shoreline length and A_0 is lake surface area.

  • Higher indices indicate higher productivity due to increased littoral zones contributing nutrients.

Unique Properties of Reservoirs

• Definition and Characteristics:

  • Reservoirs are often formed by damming rivers and can be deeper near the dam, typically becoming shallower toward delta regions.

  • Shallow Mean Depth: Often leads to increased mixing which can affect stratification.

  • Reservoirs develop a dendritic shape as they fill former stream channels, leading to a high shoreline development index.

• Biological Characteristics:

  • Larger reservoirs often show lake-like conditions deeper away from the inflow, which can lead to a change in species composition from riverine to lacustrine.

  • Each reservoir fills from the top downwards, collecting sediment and organic matter over time, leading to eventual filling.

Stratification of Lakes

• Factors Influencing Stratification:

  • Influenced primarily by temperature and salinity which lead to density differences.

  • Typical stratification begins in spring, transitioning through various seasonal phases:

  • Isothermal: Water mixes completely in spring.

  • Thermal Stratification: Formation of distinct layers (epilimnion, metalimnion/thermocline, hypolimnion) during summer.

• Fall and Winter Dynamics:

  • During fall, the cooling of surface waters initiates mixing again.

  • Winter stratification occurs when ice forms on the surface, leading to stable hypolimnetic conditions.

• Anoxia and Biogeochemical Implications:

  • Anoxia is a risk in stratified lakes during summer, leading to altered biogeochemistry and productivity.

• Mixing Regimes:

  • Lakes can be classified by how often they mix:

  • Dimictic: Mix twice per year.

  • Monomictic: Mix once per year.

  • Amictic/Meromictic: Rarely mix.

  • Polymictic: Mix multiple times per year.

Water Movement and Currents in Lakes

• Wind and Its Effects:

  • Wind creates waves which influence surface mixing and shore erosion; wave size relates to wind strength and length of the fetch.

• Langmuir Circulation:

  • Wind-induced water movement results in spiral patterns (Langmuir cells), impacting nutrient distribution and primary production.

• Seiches:

  • Occur when sustained winds pile water, creating oscillations in water levels once the wind subsides.

• Internal Waves:

  • Occur in stratified lakes under sustained winds, leading to nutrient mixing from hypolimnetic to epilimnetic waters.

Summary of Key Concepts

1. Various processes form lakes including tectonic, glacial, fluvial, volcanic and damming.

2. Lake basin morphology describes various parameters such as depth, area, and nutrient inputs influencing productivity.

3. Reservoirs are influenced by human activities and exhibit unique hydrology compared to natural lakes.

4. Wind influences lakes significantly, causing waves and currents that affect ecological dynamics.

5. Stratification patterns dictate ecological processes and productivity within the lake ecosystem.

Questions for Thought

1. Why is it difficult to assign a single geological explanation to a lake's origin?

2. How does shoreline development relate to wave size in a given lake?

3. Why do more lakes exist further from the equator?

4. Rank lakes by their average depth ratio.

5. Where are particles concentrated due to Langmuir circulation?

6. Under what conditions does thermal stratification lead to anoxia?

7. Explain how inflows into lakes interact with stratified layers.