A flood is the result of the two most basic properties of water (or
any other liquid): It flows from a place of higher elevation to a
place of lower elevation, and it spreads out to cover the bottom
and sides of whatever it is contained in. When a volume of water
is too large to be held within a container, the overflow does just
that - it flows over and upon the AREA adjacent to the container.

A flood can occur after a period of heavy rain in a low-lying area
or an AREA that is lower in elevation than the land around it. In temperate
or colder climates during late winter and early spring, a
flood can also result when the temperature is unusually high for
several days in a row. The heat wave causes the snow cover to
melt rapidly, producing water at a rate faster than the ground’s
ability to absorb it. Floods of this sort can produce enormous
amounts of overflowing water, as the accumulated solid precipitation
of the winter months is abruptly converted to liquid -- and
the flood is intensified if the ground under the snow cover is frozen
or partially frozen, since this further reduces its ability to soak
up the water.

Usually, flooding will only occur in an AREA adjacent to a river or
a lake that is fed by rivers. The excess rainfall || melted snow
flows from higher elevation to lower elevation, eventually finding
its way into a river or lake (which is as low as it can go). But that
container isn’t large enough to hold the extra volume, so the water
ends up spilling out over the river bank or lake shore and inundating
an area of land that is not normally underwater.

FLASH FLOODS:

A flash flood occurs for the same reasons that a regular flood
does, but its nature and its effects are quite different. A flash flood
is a fleeting (and fleet) phenomenon; a mass of water in a flash
flood may pass through an area in minutes, in contrast to a regular
flood that builds slowly and usually inundates an area for several
days before beginning to recede.

Flash floods occur most often in mountainous or hilly terrain after
a heavy rain, in locations where a natural body of water (a river
or a lake) is not present. The rainfall runs down slopes and accumulates
in low spots, then continues to seek even lower elevation.
As the water moves lower and lower on its journey down a
mountain or hillside, it increases volume and picks up speed. A
flash flood that began as a trickle at an elevation of 10,000 feet
can be a rushing torrent by the time it descends to 5,000 feet -
thousands of gallons of water moving forward and down at a
speed of 30 to 50 miles per hour. When a tidal wave (see the preceding
section) rushes inland, it produces effects very similar to
those of a flash flood.

The only way to avoid a flood is to move away from the affected
AREA before it becomes inundated. It’s not difficult to avoid a normal
flood, since the water level in the flooded area doesn’t increase
abruptly. But it’s a bad idea to travel through or make
camp in a mountain pass or a gully if a storm is brewing or if it has
rained within the last two or three hours; a flash flood can come
rushing through such an area with very little warning to characters
or creatures in its path.

Common sense and circumstances will dictate what happens
to a character caught in a flash flood. If he has proficiency in
swimming, he can probably avoid drowning, but he may still suffer
damage from being slammed against rocks, trees, etc., by the
onrushing water. A character without proficiency in swimming
can survive by treading water or holding his breath, but will need
to find a place of refuge before he tires. If the path of the water
carries a character past outcroppings and other protrusions, he
may be able to grab something and hold on until the flood moves
past that location. If such protrusions are available, a successful
Strength Check indicates that the character has managed to
latch on to something. Failure on this check indicates that the
character has been battered against the protrusion he was trying
to grab, and he will suffer ld3 to ld6 points of damage (depending
on thevelocityof the water and the nature of the protrusion). If
this initial Strength Check succeeds, the character must make
another Strength Check every two rounds for as long as the water
continues to rush past; failure indicates that the force of the water
has pulled his grasp loose, and he will again be carried downstream
until the velocity of the flood abates or until he is able to
grab another protrusion. With reference to any particular point
along its path, a flash flood will last for 10-15 rounds (d6 +9). After
the flood passes, any level area it has moved through will still be
covered to some depth by gently flowing water for the next 3d8
hours.