Download Watersheds - Net Start Class

Watersheds (7.8C)
Student Expectation
The student is expected to model the effects of human activity on groundwater and
surface water in a watershed.
Key Concepts
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Key Concept 1: A watershed is an area of land from which water flows into a
larger body of water such as a river, lake, stream, ocean, or aquifer.
Key Concept 2: Groundwater is obtained from aquifers, which are areas located
underground where water is stored in the pore spaces of soil and rock.
Key Concept 3: Human activity can contaminate water resources in a variety of
ways. Excess nutrients, such as nitrogen and phosphorous found in fertilizers and
human waste, can lead to large algal blooms, which quickly lower the oxygen
levels of a body of water creating dead zones. Excessive pumping of groundwater
from aquifers can cause subsidence, or sinking of the Earth’s surface.
Key Concept 4: There are two classifications for human water pollution: point
source and non-point source. With point source pollution, the source of chemicals
or other contaminants is apparent, such as a wastewater treatment plant or a
factory that releases waste directly into the water. With non-point source
pollution, no single source of pollution is evident, but contaminants still exist,
such as fertilizer or pet waste running off from suburban yards and into storm
drains.
Teacher Background
A resource document which gives teachers relevant and essential background knowledge
on the science concept being addressed.
A watershed, also known as a drainage basin, is the area of land that funnels all of its
surface water and groundwater into a body of water, such as a stream, river, lake, aquifer
or ocean. Areas of high ground separate watersheds from each other. On the large scale,
the area of land bordered by the Rocky Mountains on the west and the Appalachian
Mountains on the east is a watershed that drains all of its water into the Gulf of Mexico.
Large watersheds, such as this one, are made up of thousands of smaller watersheds.
Harris County has seven different watersheds, which help to drain all of the county’s
surface water into the Gulf of Mexico. The city of Houston is located within the Brazos
Watershed. All of the surface water in Houston eventually drains into the Brazos River,
which then drains its water into the Gulf of Mexico.
The water that is held underneath Earth’s surface is called groundwater. Surface water
can become groundwater by infiltrating, or sinking, below the surface. Groundwater can
be stored in and move through layers of rock known as aquifers. A rock layer is a good
aquifer if it has high porosity and high permeability. High porosity means that there are
many pores, or spaces, in the rock where water can reside. High permeability means that
these pores are well connected, allowing water to flow through the rock. For example, a
cupful of gravel has high porosity and high permeability, so water will flow through it
quickly. Relative to gravel, a cupful of sand has lower porosity and permeability, and
therefore, water will flow through it more slowly than through gravel.
One of the ways we get drinking water is by drilling wells into aquifers and pumping the
water to the surface. Unfortunately, we can deplete our drinking water supplies if we
pump water out faster than rain can infiltrate the ground and replenish it. During
summers in Texas, we can experience droughts severe enough that our aquifers cannot be
replenished fast enough. In order to maintain groundwater during droughts, local
governments try to regulate usage by limiting the amount of water we pump out. Also,
when we pump out too much water, the pore spaces in the rocks, which are used to hold
water, shrink in size. This shrinkage leads to subsidence, or the sinking of Earth’s
surface. If Earth’s surface sinks, our buildings and homes can be damaged (for example,
cracked foundations).
Human activities can also lead to the pollution of Earth’s groundwater and surface
waters. Water pollution is either point source or non-point source. If water pollution is
point source, we can trace back to the source of pollution (for example, a factory that is
releasing chemicals directly into a river). If water pollution is non-point source, there is
not an apparent single source of pollution. Examples of non-point source pollution
include runoff of the following pollutants into storm drains: fertilizer, pesticide,
herbicide, pet waste, and oil leakage from cars.
Water pollution can impact our health as well as the health of the environment. If there
are chemicals in our drinking water, they can lead to illness. Similarly, polluted waters
can hurt the environment. Fertilizers that are applied on farms can be washed into our
streams and rivers. If too much fertilizer is applied, eventually these contaminants drain
into larger bodies of water (lakes and oceans). The nutrients found in fertilizers (nitrogen
and phosphorus) can cause the algae in these waters to undergo a population explosion,
known as an algal bloom. When these organisms die and decompose, the oxygen in the
water is removed, leading to a dead zone. Areas of water with no oxygen are called dead
zones because living things, such as fish and turtles, cannot survive there. This situation
is now occurring in the Gulf of Mexico. The extra fertilizers being applied to farmlands
along the Mississippi River are washing into the Mississippi River and draining into the
Gulf of Mexico. The algal blooms, which occur because of this contaminated runoff,
have led to a low oxygen environment, or dead zone, in the Gulf of Mexico. In order to
ensure we have enough clean drinking water for future generations, it is important that
we use our water in a sustainable manner and that we stop water pollution.
Due to the connected processes of weathering, erosion, and deposition, Earth’s surface is
constantly changing. Weathering is the process of rock breaking down into smaller
pieces. Erosion moves weathered pieces of rock to another place. After material has been
eroded and is no longer being moved, it is deposited in a new location. In addition, once
this material has been eroded, fresh rock is exposed, which can then also be weathered.
These processes not only shape Earth’s surface (i.e. topography), but also influence soil
characteristics. Soil is made up of broken down rocks, produced by weathering and
erosion, as well as of organic matter produced by organisms.
Weathering is the process of either chemically or physically breaking down rock. If a
rock is chemically weathered, it is chemically altered or dissolved. Air and water are
agents of chemical weathering. Water can dissolve and remove minerals from rocks.
When carbon dioxide in the air dissolves in rainwater, it acts as a mild acid and degrades
the rocks that it contacts. Chemical weathering can weaken rocks and make them more
susceptible to physical weathering, but these two processes do not always occur together.
If a rock is physically weathered, it is broken into smaller pieces through mechanical
processes. For instance, the movement of the crust due to plate tectonics can cause
fractures in rocks, breaking them into smaller parts. Frost wedging is another physical
weathering process where water seeps into cracks in rock, then freezes and expands,
which makes the cracks larger. As this process is repeated over a winter or over years, the
cracks get bigger and bigger until the rock breaks into multiple pieces. In addition,
organisms cause both chemical and physical weathering. Bacteria and algae produce acid,
and when they live in cracks in rocks, this acid acts as a chemical weathering agent. Trees
and animals can act as physical weathering agents. Tree roots can grow into rocks and
break them apart. We can see this when we walk over sidewalks that have been cracked
by the trees growing next to them. Animals can also break rocks through burrowing into
them. Once rocks have been broken into smaller pieces, or sediment, either through
chemical and/or physical weathering, they can then be moved to a new location.
Erosion is the process of moving rock or sediment from one area to another. There are
several different forces that can act as agents of erosion. These include water, wind, ice,
and gravity. The strength of water, wind, and ice varies. The more powerful these
erosional agents, the bigger the pieces of rocks they move. For example, a slight breeze
can blow dirt and sand around, but hurricane-force winds can move boulders. Typically,
water in rivers moves sediment from upstream to downstream and the water usually
deposits that sediment into bays, gulfs, or oceans. Wind moves sediment along Earth’s
surface and is an important erosional agent in flat areas, such as in deserts or on the
plains. Ice is one of the most powerful erosional agents. A glacier can drag large boulders
under its ice and deposit the boulders hundreds to thousands of miles away. Gravity
moves rocks and sediments downhill. Landslides and mudslides are erosional events
where rocks and sediments are moved from a high elevation to a low elevation (due to
gravity). Eroded materials are deposited when gravity overcomes the force of the
erosional agent. For example, in a river, sediments deposit when the force of the moving
water can no longer overcome the force of gravity.
Weathering, erosion, and deposition lead to changes in Earth’s surface. Scientists have
many tools at their disposal to keep track of these changes. Topographic maps show
changes to the elevation in an area. Elevation can be continuously measured, which
allows scientists to monitor how the elevation of an area changes over time. For example,
using a topographic map made ten years ago, scientists can re-measure the elevation of a
mountain on all sides and determine if one side of the mountain is being weathered and
eroded or not. Satellite technology also allows scientists to monitor how Earth’s surface
is changing. Once a year, Scientists can direct satellites to pass over the same area on
Earth’s surface and take a photograph of the area, allowing the scientists to measure how
the surface changes with time. They can observe mountains growing or shrinking,
glaciers retreating, and rivers changing shape and direction. Knowing how Earth’s
surface has changed in the past allows scientists to predict how Earth’s surface may
change in the future - due to the processes of weathering, erosion, and deposition.