# Presentation: 348333

## Data Collection at U.S. Geological Survey Stream Gages

- Antietam Creek nr Sharpsburg, Md.

**Notes:**
U.S. Geological Survey stream gages provide hydrologic information needed to define, use and manage the Nation’s water resources. The following presentation describes methods used to collect data at USGS gaging stations. These methods, along with the underlying philosophy of the stream-gaging program, have allowed the USGS to collect relevant streamflow data for the last 100 years. 

## The first USGS gaging station was established on the Rio Grande near Embudo, New Mexico in 1889.

**Notes:**
The U.S. Geological Survey began collecting streamflow information in 1889, when the first stream-gaging station was established on the Rio Grande River near Embudo, New Mexico. 
Personnel from the Irrigation Survey, which was then a branch of the U.S. Geological Survey, were directed by John Wesley Powell to develop procedures that could be used to produce reliable streamflow estimates. Powell felt that it was  important to inventory the flow of streams in the arid west prior to settlement. 

## Embudo trainees dispersed to collect data in other locations

**Notes:**
As soon as methods were developed at Embudo, personnel were dispersed to collect streamflow data at other western locations. Within 2 years, the first streamflow measurement in the East was made on the Potomac River at Chain Bridge, near Washington, D.C., and a gaging station was established there on May 1, 1891. By 1895, discharge measurements were being made by USGS in at least 27 states.

## Streamflow Measurements in the East

- “Flood Crest March 18, 1936”

- Potomac River at Chain Bridge nr Washington, D.C.

- Potomac River at Point of Rocks, Md.

- 1889

- 1994

**Notes:**
In February 1895, a stream-gaging station was established on the Potomac River at Point of Rocks, Md. The Point of Rocks station was among the first established by USGS, and has operated without interruption since 1895. It has the longest continuous period of record of any USGS stream gage.

## The Embudo LegacyData collected using nationally consistent, prescribed standards

**Notes:**
Ever since  hydrographers left Embudo, the USGS has collected streamflow data using nationally consistent methods. These methods have allowed the USGS to collect data to the highest practical standards. All methods used to collect streamflow data are documented in USGS publications, such as the Techniques of Water-Resources Investigations series. Many USGS procedures form the basis for handbooks issued by the International Standards Organization.

## The U.S. Geological Survey currently collects streamflow data at over 7,000 sites

- .

- Potomac River at Little Falls nr Washington, D.C.

- Minebank Run at Loch Raven, Md.

**Notes:**
The USGS currently operates over 7000 gaging stations. Most stream-gaging stations are operated in cooperation with, and with funding from, federal, state, tribal, and local government agencies.

## Data from over 18,500 locations  can be analyzed with confidence

**Notes:**
In addition to current stations, USGS data bases contain mean daily-discharge data for over 18,500 locations. Because data have always been collected to nationally consistent standards all data in USGS data bases can be used and analyzed with confidence to meet both current and long-term needs.

Because it is so important for data to be collected and processed using prescribed standards, it is worth describing how data are collected at USGS gaging stations. 

## The basic piece of data collected at gaging stations is stream STAGE (water level)

- The basic piece of data collected at gaging stations is stream STAGE (water level)

**Notes:**
The basic piece of data collected at gaging stations is stream stage, which is the height of the water surface above some datum. 

## Stage can be sensed usingSTILLING WELLS

- Outside reference gages

- Intakes

- Little Patuxent River nr Guilford, Md.

**Notes:**
Stage can be sensed inside stilling wells, which are used to dampen momentary fluctuations in the water surface. The wells are connected to the stream by intakes, which allow water in the well to stand at the same stage as water in the stream. The water surface, or stage, is sensed and recorded using floats in the stilling well. Outside reference gages are used to verify that the water level in the well is the same as the water level in the stream. 

## Stage can also be sensed using PRESSURE TRANSDUCERS

- Stage can also be sensed using PRESSURE TRANSDUCERS

- Orifice Line

- Gwynns Falls at Washington Blvd. at Baltimore, Md.

**Notes:**
Stage can also be sensed using pressure transducers. The pressure exerted by water at any point in the water column is a function of water depth. This pressure can be transmitted through a gas- or liquid-filled orifice line to a pressure transducer. The USGS usually uses orifice lines filled with nitrogen gas and pressure transducers that meet USGS specification to sense pressure. Such installations are used when stilling wells are difficult or impractical to install. 

## Stage is usually:

- Measured to an accuracy standard of:
- “either 0.01 ft or 0.2 percent of the effective stage being measured” -- Office of Surface Water memorandum 93.07

- Recorded every 15 minutes

**Notes:**
According to USGS policy, installations and equipment should be capable of measuring stage to plus or minus 0.01 feet or 0.2 percent of the effective stage being measured. 

Stage is recorded every 15 minutes at most gaging stations. These data are commonly referred to as “unit values”.  

## Most users of streamflow information need to know the DISCHARGE of the stream

**Notes:**
Although stream stage is directly useful for some purposes, such as floodplain mapping, most users of streamflow information need to know the discharge of the stream. Discharge is the volume of flow passing a specific point in a given amount of time. 

## Discharge usually measured using the velocity-area method 

- Discharge= Velocity x Depth x Width

**Notes:**
The most practical method of measuring stream discharge is through the velocity-area method. Discharge is determined as the product of the cross-sectional area of the water times velocity. Measuring the average velocity of an entire cross section is impractical, so the USGS uses what’s called the mid-section velocity-area method. Using this method, the width of the stream is divided into a number of increments, each usually containing no more than 5% of the total discharge. For each incremental width, stream depth and average velocity are measured. The current meter is placed at a depth where average velocity is expected to occur. For shallow sections, this is at 0.6 of the distance from the water surface to the streambed. When depths are large, the average velocity is best estimated by measuring velocity at 0.2 and 0.8 the distance from the water surface to the streambed. The product of velocity, depth and width of the section is the discharge through that increment of the cross section. The total of the incremental discharges equals the discharge of the stream. 

## Discharge measurements are used to develop rating curves

- <date/time>

_**Rating Curve**_

_**Discharge Measurements**_

**Notes:**
A continuous record of discharge is determined by developing a relationship between stage, which is measured continuously, and discharge. This “rating” is developed using data from individual discharge measurements.

## Discharge must be measured at all stages

**Notes:**
To properly develop ratings, discharge must be measured at all stages. Otherwise, the relation between stage and discharge will be uncertain for some ranges in stage. USGS personnel visit gaging stations regularly, making discharge measurements when needed to define ratings. 

## Discharge measurements during high stages are particularly important

**Notes:**
It is particularly important to make discharge measurements when stream stages are high. Such measurements define the upper ends of ratings. As such, they form the basis for flood warning, flood forecasting and estimates of annual flow volumes. The means of making current meter measurements are provided at most USGS gaging stations. Measurements during high stages are often made from bridges or cableways. 

Sometimes, it is impossible to make current-meter measurements during large floods. When direct current-meter measurements cannot be made, discharge is measured indirectly by surveying the high-water marks left by the flood and using hydraulic formulas to calculate discharge associated with the peak stage. 

## To maintain ratings, discharge must be measured regularly

**1**

**10**

**100**

**1**

**10**

**100**

**1000**

**10000**

**Discharge (cfs)**

**Stage  (ft)**

- For example: rating changes as channel fills.

**Notes:**
Use of ratings to convert stage to discharge would be rather straightforward except for the fact that the stream channels change as sediment is eroded and deposited. For example, if sediment is deposited in the channel bottom, as indicated here in red, discharge will be less than it was for the same stage prior to when the sediment was deposited. 

Because most channels erode or fill naturally, discharge must be measured regularly. These measurements document any changes, or lack of changes, to ratings. Discharge is measured at most gaging stations every 6 or 8 weeks. Measurements are made more frequently at sites where the streambed is particularly unstable. 

## Datum must be checked regularly

- Gage houses settle

**Notes:**
Stage is the level of the stream above some datum. It is, therefore,  important that datum does not change. It is not uncommon, however, for gage structures and staff gages to be damaged by high flows and/or settle over time. If datum changes go unrecognized, incorrect estimates of discharge will result. 
Three or more benchmarks, or reference marks, are maintained at all USGS gaging stations to provide checks on datum. Level surveys are run between benchmarks, reference marks, staff gages and gage structures every few years. Level surveys are run more regularly at sites known to be unstable. 

## All gaging station data are checked and reviewed regularly

**Notes:**
All data collected at USGS Gaging Stations are checked and reviewed to insure that data have been collected and analyzed according to standards documented in USGS reports and technical memorandums. 

National consistency is asssured by tri-annual review by teams sent from USGS headquarters. The review process insures that all data published and entered into USGS data bases are collected and analyzed using prescribed national standards. 

## Data from USGS gaging stations can be used to meet a variety of  needs

**Notes:**
Streamflow information can be used to meet a variety of needs. The USGS locates and operates gages to maximize the usefullness of  data collected at gaging stations.

## Current needs

- Chesapeake Bay inflow
- Susquehanna River at Conowingo, Md.

**Notes:**
Data collected at gaging stations are used to meet  both current and long-term needs. For example, gages around the Chesapeake Bay are used to quantify freshwater inflow and to compute pollutant loadings. Likewise, gages  located along major streams are often used for input to flood-warning models. 

## Long-term needs

**Notes:**
Deciding where to locate gages to collect data to meet long-term needs is more difficult than deciding where to locate gages to meet current needs. Long-term needs, such as future bridges and flood-control projects,. require that gages be located at sites from which  data can be transferred with confidence throughout a hydrographic region. Such transfer of information can result in regional relations, such as  shown here.

Gaging stations programs are often evaluated to see how well they are meeting both long-term and short-term needs. A well designed program is capable of providing a great deal of regional information.

## Uses of Streamflow Information:Flood Frequency Analysis

- <date/time>

- Uses of Streamflow Information:_Flood Frequency Analysis_

- _Rank__	__Year__	__Peak_
-    1	1993   10,500 cfs
-    2	1995	3,200
-    3	1988	2,340
-    4	1989	1,850
-    5	1997	1,230
-    6	1990	   948
-    7	1992	   550
-    8	1996	   320
-    9	1991	   200
-  10	1994	   163

**100-year**
**flood**

**Notes:**
One of the major long-term needs met using  streamflow data  is flood frequency analysis. Annual peaks discharges recorded at gaging stations are used in statistical procedures to estimate  design discharges, such as the so-called 100-year flood. This is actually the discharge that has a 1% probablility of occurring in any one year.

The 100-year flood is commonly used as the design discharge for construction of bridges, flood channels, and delineation of flood-prone areas, such as on FEMA or flood insurance maps.

## Data dissemination

- http://water.usgs.gov
- http://md.usgs.gov

**Notes:**
Currently, daily-discharge data are published on a water-year basis for each State. In addition real-time and historic data are served on the World-Wide Web. 

## Data dissemination

**Hydrologic        **
**Stations**

**GOES**

**Wallops, Virginia**
**Command and Data**
**Acquisition Center       **

**DOMSAT**

**User Operations**

**( Internet )**

**     ****Stage     **

**http://water.usgs.gov**

**Notes:**
Data are currently telemetered from over 4000 USGS stream gages nationwide, and from about 30 gages in Maryland and Delaware. Data from most of these stations are telemetered using the GOES satellite system. Data from some sites are telemetered using FM radios, regular telephones, or cellular telephones. Data telemetered to USGS offices are generally made available to cooperating agencies. Most telemetered data are also available via the World-Wide Web.

Historic daily discharge data and instantaneous peak-flow data are also available via the World-Wide Web. These data can be downloaded and used to meet long-term needs.

## USGS continues to pursue use of new technology

**Notes:**
The USGS continues to look for new technology that will improve streamflow data collection. For example, the efficient post-mounted gage house show here makes use of newer, smaller pressure transducers and data recorders. Acoustic-doppler technology allows rapid measurement of flow in large rivers. The newly developed  Aquacalc automates data collection during current-meter discharge measurements. 

We will continue to build upon the legacy started with the Embudo station. We hope to find even more ways to collect and disseminate better data more efficiently without jeopardizing the broad-based impartial data currently found in USGS data bases.