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Hydrologic Cycle - Keynote pdf
Rain - Keynote pdf
Streams - Keynote pdf
Geomorphic Cycle - Keynote pdf
Amphitheater-Headed Valleys - Keynote pdf

Streams

Hydrologic Cycle

97% of all water on Earth is in the ocean.
2% of the rest is in the polar ice caps.

Hydrologic cycle - cycling of water in the hydrosphere.

Water is constantly entering ocean through river and ground water runoff, so why hasn’t the ocean overflowed?
Water entering the ocean is recycled seawater:

  1. Water primarily removed from sea by evaporation
  2. 90% returns to ocean as rain
  3. 10% precipitates on land and flows back to ocean

Then the cycle begins again.
Water flowing back to the ocean is called runoff.

Rain

The pattern of precipitation in Hawaii demonstrates the relationship between ascending and descending air, and precipitation and evaporation.

The windward side of a Hawaiian island is wet and the leeward side is dry.
Much of the rain in Hawai'i is orographic precipitation, or rain caused by air forced to ascend over mountains.
Warm, wet Northeast Trade Winds hit the windward side of a volcano.
Warm air holds more moisture than cold air.
The wind is forced to ascend.
Air expands and cools as it ascends, which results in precipitation, as cool air holds less moisture than warm air.
As the now cool, drier air descends over the leeward slopes of a volcano, it contracts and warms and causes evaporation.

On O'ahu, Wai'anae is extremely dry, because the process repeats as air passes over the Wai'anae shield volcano.

Streams

Rainfall is abundant in Hawaii and averages 190 cm/yr.
However, Hawaii has few perennial streams.
Perennial streams flow year round.
Streams in Hawaii tend to be ephemeral.
Ephemeral streams flow intermittently and the rest of the time they are dry.

Ephemeral streams are common in Hawaii for two reasons:

  1. The rocks in Hawaii have high infiltration rates
  2. The steep watershed encourage rapid runoff rates

High permeabilities of the rock and soil cover in Hawaii favor rapid infiltration.
Permeability - measure of a material's ability to transmit fluids.
Therefore, scientists observe little surface runoff during times of low and moderate rainfall.

Whereas the steep watersheds favor high rates of surface runoff during periods of heavy rainfall.

Even though stream flow is relatively small during most months, large amounts of erosion result from a few large storms each year.

Geomorphic Cycle

The primary processes that determine erosional topography are

Landscapes undergoing stream erosion generally go through a series of stages.
Geomorphic cycle of valley formation in Hawaii is divided into three stages:

  1. Youth
  2. Maturity
  3. Old age

Youth Stage

Young valleys are V-shaped with steep valley walls.
The stream occupies most of the narrow valley floor.
Stream erosion is greater than deposition.
A region with numerous young valleys is Hamakua coast of the Big Island of Hawaii.

As stream erosion cut to near sea level, vertical erosion slows, deposition increases, and erosion of the valley walls begins to dominate.

Mature Stage

Over time the V-shaped young valleys are widened by erosion.
Valley profiles become U-shaped.
Valleys are termed mature when they have steep valley walls separated by wide, flat valley floors.
The streams in mature valleys occupy only part of the valley floor and meander from side to side of the valley.
A region with numerous mature valleys is leeward Ko'olau.
Stream erosion equals deposition.
Mature valleys include Nu'uanu, Palolo, Kalihi, and Manoa.

With continued erosion of the valley walls, the valleys become very wide and are separated by low ridges.

Old Age Stage

With additional erosion, mature valleys become very wide and the intervening ridges are reduced in size.
Streams meander across the valley floors.
Regions with numerous old age stage valleys are leeward Wai'anae and windward Ko'olau.
Stream deposition is greater than erosion.
Examples are Lua'lua'lei Valley in Wai'anae and the coastal plain of Kailua and Kane'ohe.

Stream erosion can only erode land to sea level.
Wave erosion will erode the islands below sea level.

Amphitheater-Headed Valleys

Mature valleys in Hawaii, like Manoa and Palolo, tend to assume an amphitheater shape and are called amphitheater-headed valleys.
Amphitheater-headed valleys are wider in the head (upslope) region and narrower in the mouth (downslope) region.

The head region has more annual precipitation and more numerous streams.
The many streams run down the valley walls causing more erosion, which widens the head region.
These valleys are called vertical valleys, because the run vertically down the valley walls.

Eventually the streams coalesce into the main valley stream, e.g. Manoa Stream, which cuts primarily on the valley floor.
Additionally, the lower rainfall in the mouth region results in less erosion of the valley walls.

With a greater degree of erosion in the head region and less in the mouth region, the valley assumes an amphitheater-headed shape.

Furthermore, the greater degree of erosion in the head regions means the adjacent valleys "touch" in the head region first, forming a knife-edged ridge segment.
The lesser degree of erosion in the mouth regions results in the formation of a triangular-shaped ridge portion in this region.

The final exam notes are continued with Coastal Geology

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