To accomplish these tasks, the USGS has over 150 field offices where personnel are involved in the following activities: ♦ Collecting water samples from ground- and surface-water bodies
Stream stage is an important concept when analyzing how much water is moving in a stream at any given moment. Stage is the water level above some arbitrary point in the river and is commonly measured in feet. For example, on a normal day when no rain has fallen for a while, a river might have "a stage of 2 feet." If a big storm hits, the river stage could rise to 15 or 20 feet, sometimes very quickly. This is important because, from past records, we might know that when the stage hits 21 feet, the water will start flowing over its banks and into the basements of houses along the river -- time to tell those people to move out! How high and how fast a river will rise during a storm depends on many things. Most important, of course, is how much rain is falling. But also we have to look at other things, such as the stage of the river when the storm begins, at what the soil is like in the drainage basin where it is raining (is the soil already saturated with water from a previous storm?), and at how hard and in what parts of the basin the rain is falling. The USGS has studied these things at many places across the country for many years, and thus is often able to make predictions about if and where a flood will occur and how bad that flood will be. With the advent of modern computer and satellite technology, the USGS can monitor the stage of many streams almost instantly. Since some streams, especially those in the normally arid Western U.S., can rise dramatically in a matter of minutes during a major storm, it is important to be able to remotely monitor how fast water is rising "in real time" in order to warn people that might be affected by a dangerous flood. Recreational users of streams, such as kayakers, also use "real-time" stream-stage data to tell them if certain streams are at the right height for kayaking. The USGS can now gather data on stream stage and even produce graphs showing stage as the rain is falling. In fact, some of these real-time data and graphics are being made available for you to use via the World Wide Web. You can access current stream conditions for your state right now.
Not directly. You cannot say that because a stream rises (doubles) from a 10-foot stage to a 20-foot stage that the amount of water flowing also doubles. Think of a cereal bowl with a rounded bottom. Pour one inch of milk in it. It doesn't take much milk to make it up to the one inch level because the bowl is least wide near the bottom. Now, pour in milk until it is two inches deep -- it takes a lot more milk than it did to fill the first inch because the bowl gets wider as you go up. The same thing happens in a stream -- the stream banks will generally be narrower at the bottom and tend to widen as you go up the bank. So, the amount of water flowing in a stream might double when the stage rises from 1 to 2 feet of stage, but then it might quadruple when it goes from 3 to 4 feet. This graphic helps to illustrate: 
To find out how much water is flowing in a stream or river, USGS personnel have to go out and make a "discharge measurement." USGS uses the term "discharge" to refer to how much water is flowing, and discharge is usually expressed in "cubic feet per second" (think of a cube of water one foot on a side, and how many of those move past a point in one second). To do this, we often have to go out and stand in the creek, measure the depth and how fast the water is moving at many places across the creek. By doing this many, many times, and at many stream stages, over the years we can develop a relation between stream stage and discharge. Stream stages are not always cooperative, so its not uncommon for someone to have to go measure a stream at 2:00 in the morning during a storm, sometimes in freezing conditions! Also, the stream can be uncooperative in that it changes -- a big storm may come along and scour out bottom material of a creek, or lodge a big log sideways in the creek, or sometimes do both at the same time. These kind of changes result in changes in the relation between stage and discharge. 
A more detailed explanation is available.
The term "100-year flood," is used to describe the recurrence interval of floods. As the table below shows, the "100-year recurrence interval" means that a flood of that magnitude has a one percent chance of occurring in any given year. In other words, the chances that a river will flow as high as the 100-year flood stage this year is 1 in 100. Statistically, each year begins with the same 1-percent chance that a 100-year event will occur.
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But, just because a 100-year flood happened last year doesn't mean that it won't happen this year, too. In other words, future rainfall and floods don't depend on the rainfall and floods that happened in the past. The past records are mainly used to show what kind of river flows can be expected. So, when you hear about a 100-year flood, at least you have a general idea that it does mean a BIG flood, and if you hear of a 200-year flood you know that it means one even BIGGER! As an example, in July of 1994, some places in south Georgia received more than 20 inches of rainfall in a few days -- the floods they produced were tremendous... way over the 100-year flood. At Senoia, Ga., the maximum amount of water flowing by the Line Creek gage was 2.4 times greater than the 100-year flood level.
(5) Is measuring water in a well like measuring a stream?
No. In ground-water work the USGS puts a lot of effort in measuring the water levels in observation wells. Since water levels in aquifers can change (for a variety of reasons) we need to keep accurate records of these changes, and what factors affect them. Water levels in wells can definitely be affected by water withdrawals nearby -- and sometimes far away.
One way to keep a record of the water levels in a well is to place a float at the end of a wire and lower it into a well. The float will go up and down as the water in the well goes up and down. The other end of the wire is attached to a machine that has a pen-like instrument attached, and the pen point moves up and down according to the action of the float. A roll of paper slowly rolls past the pen, so a record of water level is plotted continuously on the paper. Sometimes we don't use paper -- we just log the changes straight into computer memory.
(6) What can cause water levels in wells to change?
Water levels in wells are constantly changing both in the short term and over the long term. Some wells even have a seasonal change. In the short term, water levels can be lowered just by pumping water out of the well for use. Also, a well may be pumped so much as to cause the water level in nearby wells to be lowered, too. It all depends on how fast the aquifer that the well uses is resaturated with water from the surface or from the area surrounding it (recharge). In some places people have withdrawn water faster than water replenishes the aquifer, and the wells have stopped producing water.
Sometimes this is a long-term problem occurring over a very large area. If it takes a long time to replenish the aquifer, maybe because the aquifer is composed of rock that only allows water to move through it very slowly, a field of wells may stop producing. Users will have to wait until the aquifer becomes more saturated again before turning the pumps back on. Also, an aquifer can only contain water if there is water coming into it, usually from rainwater seeping down from the surface. In a severe drought water levels in wells can significantly decline.
(7) If the ground filters water, is ground water always clean?
Water being drawn from a well was once precipitation that fell onto Earth's surface. It seeped into the ground and, over time, occupied the porous space in some subsurface material. Naturally, big particles that can be found in streams, such as leaf chunks, will not be seen in ground water. So, yes, big particles are filtered out. But ground water can contain other items that you can't see. Some are naturally occurring and some are human-made substances. Ground water can contain hydrogen sulfide or other naturally occurring chemicals. Ground water also may contain petroleum, organic compounds, or other chemicals introduced by humans' activities.
Contaminated ground water can occur if the well is located near land that is used for farming where certain kinds of chemicals are applied to crops, or near a gas station that has a leaking storage tank. Leakage from septic tanks and/or waste-disposal sites also can contaminate ground water. A septic tank can introduce bacteria to the water, and pesticides and fertilizers that seep into farmed soil can eventually end up in water drawn from a well. Or, a well might have been placed in land that was once used for something like a garbage or chemical dump site. In any case, it is wise to have your well water tested for contaminates.
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