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Race —
(1) A strong or swift current of water.
(2) The channel of such a current. Also, an artificial channel built to transport water and use its energy; a Raceway.

Raceway — A rectangular fish rearing unit that has a continuous flow of freshwater to maintain suitable oxygen, temperature, and cleanliness for intensive production.

Radial Drainage — An arrangement of stream courses in which the streams radiate outward in all directions from a central zone or inward from all directions to a central area.

Radial Flow — The flow of water in an aquifer toward a vertically oriented well.

Radioisotope — Isotopic forms of an element that exhibit radioactivity. Isotopes are varieties of a chemical element that differ in atomic weight, but are very nearly alike in chemical properties. The difference arises because the atoms of the isotopic forms of an element differ in the number of neutrons in the nucleus. For example, ordinary chlorine is a mixture of isotopes having atomic weights of 35 and 37, and the natural mixture has an atomic weight of about 35.543. Many of the elements similarly exist as mixtures of isotopes, and a great many new isotopes have been produced in the operation of nuclear devices such as the cyclotron. There are 275 isotopes of the 81 stable
elements, in addition to more than 800 radioactive isotopes.

Radionuclides — Radioactive chemicals that are usually naturally occurring and found in drinking water. Typical radionuclides for which the U.S. Environmental Protection Agency (EPA) has established Maximum Contaminant Levels (MCLs) as part of its enforcement of the Safe Drinking Water Act (SDWA) include radium 226 and 228, gross alpha particle activity, and beta particle activity.

Radius of Influence — The radial distance from the center of a well bore to the point where there is no lowering of the water table or Potentiometric Surface (the edge of its Cone of Depression).

Radon — A naturally occurring, colorless, odorless, radioactive gas formed by the disintegration of the element radium; damaging to human lungs when inhaled.

Rain Shadow — A dry region on the lee side of a topographic barrier, usually a mountain range, where the rainfall is noticeably less than on the windward side.

Rainbow — (Meteorology) A circular bow or arc exhibiting, in concentric bands of light, the several colors of the spectrum, and formed opposite the sun by the refraction and reflection of the sun’s rays in drops of rain. Also, a similar arc may be formed by the moon, or some other source of light, or one formed in spray mist, fog, clouds, etc. Rainbows are circular because the drops, being spherical, are alike in every position. In the case of the ordinary rainbow, or Primary Rainbow, the effective rays are refracted on entering each drop, reflected from its interior surface and refracted again on emerging, then passing to the observer’s eye. The red is seen on the outside edge of the bow. Also, there is often seen another larger bow, the Secondary Rainbow, concentric with and near the first. Its formation differs from that of the primary rainbow in that there are two internal reflections, it is much fainter, and it has the red on the inside edge. Faint-colored arcs sometimes seen next to the primary or secondary bow, due to interference, are Supernumerary, or Spurious, Rainbows. White Rainbows are sometimes formed by fog or clouds, but are too minute to give distinctly the concentric bands of color of the ordinary rainbow.

Random Sample — (Statistics) A sample selected in such a manner that all possible samples of the same size have an equal and independent chance of being included.

Random Variable — (Statistics) A variable characterized by random behavior in assuming its different possible values. Mathematically, it is described by its probability distribution, which specifies the possible values of a random variable together with the probability associated with each value.

Rapid Drawdown — Lowering the elevation of water against a bank faster than the bank can drain, leaving a pressure imbalance that may cause the bank to fail.

Rapids — A part of a stream where the current is moving with a greater swiftness than usual and where the water surface is broken by obstructions, but without a sufficient break in slope to form a water fall, as where the water descends over a series of small steps. It commonly results from a sudden steepening of the stream gradient, from the presence of a restricted channel, or from the unequal resistance of the successive rocks traversed by the stream.

Rare Species — A species of plant or animal which, although not presently threatened with extinction, is in such small numbers throughout its range that it may be endangered if its environment worsens.

Rate Structures (Water and Wastewater Treatment) — Rate structures for water and wastewater treatment are generally classified into three primary categories: declining block, uniform, and inverted. Details of these primary rate structures are as follows:
[1] Declining Block Rate — Provides a means of recovering costs from the customer classes under a single rate schedule, recognizing the different water and wastewater demands and costs associated with each customer class. Under this rate schedule economies of scale are recognized since the price per unit declines as the water customer consumes more water;
[2] Uniform Rate — Provides separate rates for each customer class based on the demand, use, and other characteristics of the customer class;
[3] Inverted-Block Rate — Incorporates a unit charge that increases with increasing water consumption or demands for wastewater treatment; requires a multiple blocking structure with the rate per unit of consumption increasing with each successive level. Variations and applications of these primary rate structures typically include:
....[1] Unmeasured Usage — Flat rate, irrespective of usage, generally based on (pipe) size of ....service;
....[2] Constant Block with Service Charge — Constant rate per unit, times the number of units ....consumed, plus flat rate service charge;
....[3] Increasing Block with Service Charge — Cost per unit increases with increasing (block) rate of ....water usage, plus flat rate service charge;
....[4] Decreasing Block with Service Charge — Cost per unit decreases with increasing (block) rate ....of water usage, plus flat rate service charge;
....[5] Increasing Block with Minimum Allowance — Cost per unit increases with increasing (block) rate ....of water usage over a minimum charge for minimum amount of use (Lifeline Rate).

Rating Curve —
(1) A graphic representation of a Rating Table (see below).
(2) A curve showing the relation between Gage Height and discharge of a stream or conduit at a given Gaging Station. (3) A curve showing the relation between the discharge of a gage, meter, or other hydraulic structure or instrument and the pertinent hydraulic conditions affecting the discharge, such as pressure, hydrostatic head, and velocity of approach. If more than one condition affects discharge, a family of curves is needed to represent the rating.

Rating Table —
(1) A table showing the relation between two mutually dependent quantities or variables over a given
range of magnitude.
(2) A table showing the relation between the Gage Height and the discharge of a stream or
conduit at a given Gaging Station.
(3) A table showing the relationship between the stage in a reservoir and its volume. Also referred to as Discharge Table.

Rational Method (or Formula) —
(1) A simple procedure for calculating the direct precipitation peak runoff from a watershed, using the rainfall intensity, the area of the watershed, and the runoff coefficient appropriate for the type of watershed runoff surface.
(2) A technique for estimating peak discharge rates based on average rainfall intensity (i), the drainage area (A), and a coefficient based on watershed characteristics (C). The discharge in cubic feet per second is derived from the following formula: Q = CiA. The rational method is commonly applied to areas as large as 5 square miles, but is preferably used for drainage areas under a half square mile. The 2-, 5-, 10-, 25-, and 50-year flood recurrence discharges can be estimated with this formula.

Rates of Rise and Fall — How rapidly the elevation of the water rises and falls during a flood.

Ravine — (1) A deep, narrow valley or gorge in the earth’s surface worn by running water. (2) A small narrow steepsided valley that is larger than a gully and smaller than a canyon and that is usually worn by running water.

Reach — A specified homogeneous length of a stream.

Riparian Zone / Riparian Buffer — The land adjacent to streams, rivers, and lakes that actively interfaces with the waterbody through physical and chemical processes. Healthy riparian zones filter nutrients and sediments, increase streambank stability, and provide shade that reduces stream temperatures.

Recharge — Water that infiltrates the ground and reaches the saturated zone.

Redd — Pit-like nest dug in the gravel of a stream bottom by a female fish where eggs are laid, fertilized by the male and re-covered with gravel.

Runoff — Excess rainwater or snowmelt that is transported to streams by overland flow, tile drains, or ground water.