Study Notes: Lakes and Reservoirs

CHAPTER 7: Lakes and Reservoirs: Physiography

  • Definition of Lakes:

    • A lake is defined as a very slowly flowing or non-flowing (lentic) open body of water in a depression that is not in contact with the ocean.

    • Includes saline lakes, excludes estuaries and marine embayments.

  • Characteristics:

    • The distinction between small shallow lakes (ponds) and wetlands can be unclear.

    • Lakes represent a continuum of aquatic habitats varying in depth and water velocity.

  • Occurrence:

    • Permanent lakes are typically found in areas with more precipitation and suitable geology for water retention.

    • Geological histories influence lake formation; for example, northern North America has more lakes than wetlands, while northern Asia and northeast Europe are dominated by wetlands.

    • Intermittent lakes are common in arid regions (e.g., Western United States, South Australia).

  • Human Impact:

    • Human activity has significantly altered the distribution of lentic habitats through the construction of lakes and ponds.

    • Approximately 2.6 million small impoundments exist in the contiguous United States, comprising roughly 20% of all lentic water therein.

  • Lake Distribution by Size:

    • There are vastly more small lakes than large lakes, but large lakes hold a similar total surface area to smaller lakes. Large lakes are deeper and contain significantly more water..

Lake Habitats and Morphometry

  • Classification of Lake Habitats:

    • Lake habitats are referred to as lentic or lacustrine, characterized by deep, nonflowing waters.

    • Includes various subhabitats:

    • Pelagic Zone: Open water where light penetration allows photosynthesis.

    • Profundal Zone: Benthic habitat below the pelagic waters with insufficient light for photosynthesis.

    • Littoral Zone: Shallow zone where light reaches the bottom, supporting growth of photosynthetic organisms.

  • Morphometry:

    • Refers to the size and shape of lakes, important for classifying lake types based on geomorphological properties.

    • Key measurements:

    • Surface area (A)

    • Maximum depth ($z_{max}$)

    • Mean depth ($z$)

    • Volume ($v$), calculated as:
      v=Aimeszv = A imes z

    • Shallow lakes are more productive than deeper lakes due to better nutrient mixing from bottom to surface.

    • Typical relationship: as surface area increases, mean depth also increases, but this relationship is variable.

  • Water Retention Time:

    • Determines how long water remains in the lake, calculated using the formula:
      R=vLR = \frac{v}{L}
      where $L$ is volume loss from evaporation and outflow.

    • Example: Lake Tahoe has a residence time of about 700 years.

  • Shoreline Development Index ($D_L$):

    • Quantifies the irregularity of the shoreline, calculated by comparing the actual shoreline length ($L$) to a hypothetical straight shoreline for the same surface area ($A0$): D</em>L=L2πA0D</em>L = \frac{L}{2 \sqrt{\pi A_0}}

    • Lakes with high shoreline development tend to be more productive.

  • Example Comparison:

    • Comparing Crater Lake and Milford Reservoir:

    • Crater Lake:

      • Volume: 20.3 km³, Mean Depth: 364 m

    • Milford Reservoir:

      • Volume: 0.48 km³, Mean Depth: 7.4 m

    • Productivity differences attributed to morphometric properties and watershed activities.

Unique Properties of Reservoirs

  • Definition:

    • Reservoirs are man-made lakes created for various purposes, including water storage and hydroelectric power generation.

  • Contrasts with Natural Lakes:

    • Often deepest near the dam and shallower at deltas.

    • Features a dendritic or tree-like shape, leading to high shoreline development.

    • Productivity varies, often high unless turbidity limits light.

    • Ballpark of reservoirs worldwide, sedimentation can limit operational life.

Geomorphological Evolution of Lakes and Reservoirs

  • Sedimentation:

    • Lower water velocity in lakes leads to sediment retention, forming deltas.

    • Sediment transference occurs from tributaries, wind, and organic decay, influencing lake fill and ecology.

  • Lakes’ Fates:

    • Over time, lakes can fill with sediment, becoming wetlands or meadows.

    • The evolution model states lakes transform depending on sedimentation rates and lake type.

  • Reservoir Management:

    • Practices aim to extend reservoir life by managing sediments and water quality.

Stratification

  • Temperature and Density Effects:

    • Density variations caused by temperature and salinity lead to lake stratification.

    • Classic model observed in cold-temperate lakes includes three layers:

    • Epilimnion (warm surface layer)

    • Metalimnion (thermocline; transitional layer)

    • Hypolimnion (cold, deep layer)

  • Seasonal Changes:

    • Spring: Isothermal conditions allowing complete mixing.

    • Summer: Stratification occurs; cooling empowers mixing conditions for autumn.

    • Ice cover can stabilize stratification in winter.

  • Lakes’ Mixing Regimes:

    • Descriptions include dimictic (twice yearly), monomictic (once yearly), polymictic (multiple mixings), amictic, and meromictic (no mixing).

  • Influence of Salinity:

    • Salinity can stabilize stratification layers, as seen in tropical lakes.

Advanced: Heat Budgets of Lakes

  • Definition:

    • Heat budget assesses energy required to heat a lake across a year.

    • Influences biological processes, metabolism, and climate moderation.

  • Equation for Heat Balance:
    QR+QE+QL+QV+QS+QB=0QR + QE + QL + QV + QS + QB = 0

    • Components:

    • $QR$: Radiation

    • $QE$: Latent heat

    • $QL$: Heat loss through evaporation

    • $QV$: Heat exchange via inflow/outflow

    • $QS$: Sensible heat exchange

    • $QB$: Heat exchange with sediments

  • Annual Variability:

    • Cycles of heating vary by annual climate conditions.

Water Movement and Currents in Lakes

  • Mechanisms of Movement:

    • Wind-induced waves dominate mixing and erosion in lakes.

    • Waves influenced by wind strength, duration, and fetch (the length of water wind acts on).

  • Langmuir Circulation:

    • Wind causes spiral circulation patterns with zones of upwelling/downwelling.

    • Affects positions of nutrients and floating materials in lakes.

  • Seiches:

    • Occur when wind-driven surface movement creates oscillation effects on lakes.

    • Can impact benthic organisms and nutrient distribution.

Summary

  1. Lake Formation: Processes can be tectonic, glacial, fluvial, volcanic, and damming.

  2. Morphology Parameters: Include depth, area, volume, shoreline development, and watershed area.

  3. Reservoirs: Unique habitats blending features of lakes and streams with significant ecological impacts.

  4. Waves: Heightened where fetch is maximized.

  5. Stratification: Alters circulation, affecting ecosystem dynamics.

  6. Thermal Layers: Identified as epilimnion, metalimnion, hypolimnion. Stratification affects biogeochemical cycling.

  7. Salinity: Involves similar stratifying characteristics, impacting circulation.

  8. Wind Influence: Drive Langmuir circulation and seiche phenomena leading to further mixing.