Lake Definitions and Morphology

Lake Definitions

• There is no official distinction between a lake and a pond.
• To determine if a body of water is a lake or a pond, consider if light reaches the bottom at the deepest point, if the water body is relatively uniform, and if the waves are smaller than 30 cm in height.
• Lakes and rivers are distinct, but there is overlap. Lakes are lentic and rivers are lotic ecosystems.
• Reservoirs can be a combination of lakes, rivers, and reservoirs.
• Reservoirs are artificial lakes filled by river inflow.

Lake Morphology

• Surface Area:

  • Measured using maps or remote sensing techniques.
  • Caspian Sea is the largest saltwater body.
  • Aral Sea is a shrinking saltwater body.
  • Lake Superior is the largest freshwater body.

• Depth:

  • Measured using soundings (weighted line) or sonar.
  • Bathymetry is a map of the lake bottom.
  • Mean depth (z) is the average depth of the lake.
  • Volume is calculated by multiplying surface area by mean depth or by summing the product of area and depth of horizontal segments.

• Hypsographic Curve:

  • Represents the depth-area distribution of the lake.
  • Shows the amount of each depth zone, useful for assessing biological reproduction potential.

• Bathymetric Map:

  • A map of the lake bottom topography.
  • Shows the depth contours of the lake.

Kootenay Lake

• Located in the Kootenay River Basin, which is 50,298 km2.

• The Kootenay River has a stream order of 7.

• Kootenay Lake Basin is 31,264 km2, with 174 inlets and one outlet (Kootenay River).

• Average discharge of the Kootenay River at Kootenay Lake is 782 m3/s.

• Dimensions:

  • Length: 104 km.
  • Width: 3-5 km.
  • Surface Area: 421 km2 or 421,730,000 m2.
  • Perimeter: 405 km or 405,082 m.
  • Deepest Point: 150 m.
  • Average Depth: 94 m for the main lake, 10 m for West Arm.

• Volume:

  • Calculated as Area x Mean Depth.
  • Volume = 421,730,000 m2 x 94 m = 39,642,620,000 m3.

Foreshore

• Land between the high and low watermarks, including beaches.
• In BC, foreshore is either unceded Indigenous lands or public land, not private land.

Shoreline Characteristics

• Reservoirs have jagged shorelines due to the flooding of river valleys.

Shoreline Regularity Index (SRI)

• Measures the deviation of a lake's shape from a circle.

• Minimum is 1.0 for circular lakes.

• Higher values (4+) indicate irregular shapes like reservoirs or shoestring shapes.

• High SRI and shallow depths create extensive littoral zones.

• Calculation:

  • SRI = Shoreline Length / Circumference of Area.
  • For Kootenay Lake: SRI = 405,082 m / (2√421,730,000π) = 5.56.

Hydraulic Residence Time (HRT)

• The average amount of time water spends in a lake.

• Important for pollution studies and nutrient regimes.

• Calculations:

  • HRT (years) = Volume (m3) / Q (m3/s) x 1 year / 31,536,000 s/year.
  • For Kootenay Lake: HRT = 39,642,620,000 m3 / 782 m3/s x 1 year / 31,536,000 s/year = 1.6 years.
  • Alternatively, HRT (years) = Volume (Lake) (m3) / (Basin Area (m2) x Avg. Precip (m/year) x 0.85).
  • For Kootenay Lake: HRT = 39,642,620,000 m3 / (31,264,000,000 m2 x 0.9 m x 0.85) = 1.66 years.

Light Penetration

• Photic Zone:

  • Surface down to where 1% of light penetrates.
  • Zone of net oxygen production (photosynthesis > respiration).
  • Depth depends on water clarity.

• Aphotic Zone:

  • Bottom of the photic zone to the bottom of the lake.
  • More carbon dioxide is produced than oxygen (respiration > photosynthesis).

Water Transparency

• Measured using Secchi Disk Depth.
• Secchi Disk Depth is the depth at which the disk disappears and then reappears.
• Photic Zone is roughly 2.5 times the Secchi Disk Depth.

Crater Lake

• Average Secchi Disk Depth of 1-10 meters.
• Crater Lake has a Secchi Disk Depth of 43 meters.