Continental Margins (OSS)

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Continental margins are water-covered areas that extend from the shoreline of continents down to the deep ocean floor; they include shelves, slopes, and rises. Continental margins contain much of the marine life commonly encountered by surface dwellers. Many of the underwater demihuman and humanoid races (such as locathah, scrags, sea elves, sahuagin, and tritons) make their homes here.
==Continental Shelves==
A continental shelf is a gently sloping water-covered area that connects to the shoreline and extends outward and downward (to a depth of approximately 600 feet). These shelves can be either very narrow or quite broad (800 or more miles wide) depending on the amount of seismic activity in the area a relatively inactive coastal area allows sediment to accumulate and form broad shelves.
Despite their name, continental shelves are not uniformly flat. Rather, these areas possess numerous terraces, hills, and canyons. Dungeon Masters can populate these features with all sorts of marine life—including several intelligent aquatic races (sahuagin, locathah, and so on), as well as kelp forests and coral reefs (described on the next page).
==Continental Slopes==
Continental slopes begin where continental shelves end. These slopes usually vary in width from 12 to 62 miles and end at a depth of 4,900 to 11,000 feet—well below the depth at which light penetrates the water. The downward angle of these slopes ranges from as gentle as 1 degree to as sharp as 90 degrees.
Continental slopes often possess cliffs of sedimentary rock naturally hewn by the action of faults and other geologic processes. In addition, deep V-shaped depressions, known as submarine canyons, can cut through the continental slope.
===Submarine Canyons===
These deep V-shaped valleys have irregular floors and steep walls; they often extend through the continental slope and into the continental rise.
Submarine canyons often house a fast-moving current filled with sediment. This turbidity current moves through the canyon, picking up more sediment as it speeds along its way. Characters caught within it must have some means of securing themselves to the walls or floor of the canyon, or they suffer 3d6 points of damage (from careening into the irregular stone walls and floors of the canyon) and are swept through the valley and deposited several miles away from their starting points.
The water clarity within a submarine canyon remains turbid for at least a day after the passing of a turbidity current. There is a base 10% chance per day of a turbidity current sweeping through a canyon. This chance increases to 70% after a major seismic disturbance (such as a seaquake or an underwater volcanic eruption) within 50 miles of the canyon. Spells that affect a large portion of the surrounding underwater terrain (such as earthquake or tsunami) will also trigger a turbidity current.