Surface WaterChapter 9

Section 9.1Surface Water Movement

The Earth’s water supply is recycled through a continuous process called the water cycle. Through this process, water molecules move continuously through the water cycle following several pathways:

Molecules evaporate from a body of water on surfaceMolecules condense into cloud dropletsMolecules fall as precipitation back to the Earth’s surfaceMolecules soak into the ground

Eventually, the molecule evaporates back into the atmosphere

RunoffWater that flows down slope along the

Earth’s surfaceRunoff can do a few things once it reaches

the surface:Reach a stream, river, lake, or oceanEvaporate back into the Earth’s atmosphereAccumulate into puddles on Earth’s surfaceEventually seep into the ground

Factors that Influence Runoff:Vegetation plants allow for more water to enter into the ground by holding the soil in place, and slowing down the flow

Rate of precipitation lots of rain in short periods of time lead to lots of runoff that doesn’t have a chance to soak into the ground

Soil composition if the soil has lots of large pores it will absorb water very easily. However, smaller pores make it harder for water to be absorbed

Slopemore slope means a faster water flow and thus less water to get soaked into the ground

So… where does the runoff go? Most runoff will go into stream

systems. Streams begin as small channels

(rills) and eventually turn into rivers.

All streams flow down slope, to lower elevations.

The actual path of the stream varies, with some flowing into lakes and others flowing directly into the ocean. In addition, some streams flow into other stream systems. These streams are referred to as tributaries.

Stream Systems Watershed Divide

All of the land area whose water drains into a stream system

Also referred to as a drainage basin

Can be relatively small, or extremely large in area

The Mississippi River has the largest watershed in the USA

High land area that separates one watershed from another

All of the materials that the water in a stream carries is known as the stream’s load.

Stream Water carries both living and non-living components:Living components of water include

microscopic life-forms as well as larger plants and animals.

Non-living components of surface water include sediments, dissolved solids, and dissolved atmospheric gases (oxygen)

How a Stream Carries its LoadSolution Suspension

Material is carried in a solution after it becomes dissolved in a stream’s water

How much is carried depends on the material through which the stream’s water has passed

When water runs through or over rocks containing soluble minerals, it dissolves small amounts of the minerals and carries it away in a solution

Expressed in parts per million (ppm)

All particles small enough to be held up by the turbulence of a stream’s moving water are carried in suspension

The amount of material in suspension varies with the volume and velocity of the stream water

Rapidly moving water can carry larger particles than slowly moving water can

As velocity decreases, the heavier particles settle to the bottom

BedloadSand, pebbles, and cobbles that are rolled or pushed along the bed of the stream

The faster the water moves, the larger particles it can carry in its bedload

As particles move they bump into one another, scraping and grinding against one another in a process called abrasion

This process contributes to the physical weathering of the stream’s bottom and sides, providing an additional source of erosion.

As gravity pulls stream water to lower elevations, the load moves along with it. As the river slows, the streams bedload is “dropped” until it is picked back up by faster water currents

Stream Velocity & Carrying CapacityThe ability of a stream to transport material is

referred to as its carrying capacity.

Carrying capacity is dependent upon the velocity (speed) and the amount of water moving through the stream

Discharge is the measure of volume of stream water that flows over a particular location within a given period of time.

FloodplainsA flood occurs when water spills over the sides of a stream’s banks onto adjacent lands.

This broad, flat area extending from a stream’s bank is referred to as the floodplain.

Floods are natural occurrences and continue to rise DAYS after the precipitation ends. Why? Because it takes time for runoff to collect into streams.

Section 9.2Stream DevelopmentAs a stream develops, it changes in shape, width, and size. In addition, the landscapes where the stream flows also changes.

Steps in stream development:1. An adequate supply of water must be available (usually from rain)2. The water supply must accumulate in what is called headwaters3. Falling precipitation accumulates in small gullies and begins to

move4. The moving water carves a narrow pathway called a stream channel5. In time, the channel widens and deepens, with more water

accumulation6. The water is held within the stream banks, which is the ground

bordering the stream on each side

Formation of Stream ValleysAs the stream flows, it actively erodes its path in

the shape of a “V”. It continues to erode like this until it hits the base level, then it begins eroding from the sides.

The process of erosion continues, eventually making a “U” shaped valley.

The “U” shaped valleys continue, creating bends and turns in the streams… called meanders.

Meandering StreamsA meander is made when one

side of the stream experiences less resistance than the other side.

The side that is slowed deposits sediments, the other side moves faster… eroding off the sides.

As result, a curve is created.In time, it is common for a

stream to cut off a meander and begin to flow along a straighter path. Eventually, the water supply is blocked off and the meander becomes an oxbow lake.

Oxbow lakes eventually dry up.

Deposition of SedimentsAs streams lose their velocity (speed), they lose their

ability to carry sediments. In dry regions, where mountain streams flow down

narrow valleys onto a flat, broad valley floor, the stream’s gradient suddenly decreases causing the stream to drop sediments in a fan-shaped deposit. This is called an alluvial fan.

Streams also lose velocity and the ability to carry sediment when they join larger bodies of quiet water. The triangular deposit that forms at this point is called a delta. Delta deposits typically consist of silt and clay particles

Alluvial Fan Delta

RejuvenationDuring the process of stream formation, downcutting (the

wearing away of the streambed) is a major process of erosion.

Eventually, this stops as the stream reaches its base level.However, the land below the stream can uplift (as the base

level lowers).When this occurs, rejuvenation (to make young again) is

taking place.During rejuvenation, the stream actively resumes the

process of downcutting toward its base level, increasing the rate of flow and making the stream channel, once again, become “V” shaped.

9.3Lakes and Freshwater WetlandsLake – a depression in the surface materials of a landscape that collects and holds water.

Lake Facts:Usually connected to streams, rivers, or empties into an ocean.Has a constant supply of water.Some are used as a reservoir to hold water

Formation of Lakes

Lakes form in different ways:Streams can be

cut off (oxbow lake)

Glacier remains (kettle lake)

Dissolved limestone

Man made

Lake Changes…

Lakes can undergo changes from rain, runoff, and underground sources.

EutrophicationThe process by which

lakes become rich in nutrients from the surrounding watershed, resulting in the change of the organisms in the lake

WetlandsA land area that is covered with water for a

large part of the year. Specific soil types that support some plant

speciesAlso referred to as bogs, swamps, and

marshesBogs – not stream-fed, receive water from

precipitationMarshes – Constant supply of water along the

mouths of streams and in areas with extensive deltas

Swamps – low-lying areas often located near streams, develop from marshes