The Upwelling Lake Habitat

Derek Holland
One rare form of habitat that can be found almost anywhere on land is the upwelling lake, a spring writ large. What makes the lake different from those created by rain fall or river convergence is their lower depths, which can go as far as the planet's core, and its ecology as many of the creatures are natives to the underearth or descendants thereof for worlds where evolution happens. Here are a few example lakes that use existing creatures from different editions and a few of my own.
Garven Lake is a smaller body of water, only around six hundred acres in size, and is found in a subarctic land. Its water comes mostly from the sea that sits at the bottom of the underearth below it. That water becomes fresh in its travels up to the surface, leaving behind salt deposits in side caves. As for its animal population, that is mostly dominated by hetfish (second annual Monstrous Compendium). They warm the waters enough that the intelligent inhabitants are tako (Monstrous Manual), which normally are found  in tropical waters. The octopus people have evolved from hides (Dragon Magazine) and are a source of trade between the underearth and surface communities around the lake and its upwelling river source.
Chuul (3.X Monster Manual) are masters of Lake Yadnis, a death trap found in a high valley. The lake is unusually large for such a lake at more than 10 square miles and its spill off river provides irrigation for a nation of aberrations that lives on the surface. The water is slimy and always feels chilled no matter its temperature. Sages think its source is an aboleth city in an underearth lake but no one has survived the dive to confirm this. The plants that do grow on its banks have fish scales or skin instead of bark and few humanoids find the lumber useful. And it burns very oddly, in a fractal manner that produces flames of green and purple.
Silver Lake is named for the salt deposits on its shores as they do glint silver in bright sunlight. Like Garven, the waters come from the underearth sea but stays salty for the entire trip to the surface. The waters are so salty that only few creatures can survive drinking it. Drik (second Dark Sun Monstrous Compendium) are one of the few, as are a aquatic adapted species of rust monster. In fact is the rust monsters that cause such build ups of salt on the banks- it is their waste product after filtering out all the dissolved metals from the water. Why the salt glints silver is that is does contain a tiny amount of the metal and in a form that does not tarnish. One community of people does collect the salt and separates the minerals, selling the silver to mages and royalty. Another creature found drinking from Silver Lake is the saltjacket, a species of giant wasp that uses the salt to create nests that most predators avoid for the salt is mixed in their venom.
Undsu is a lake found on an island in the middle of the ocean. A pair of rocs use the island as a roost and nest and wash in the lake, fouling its waters. The lake is populated by aquatic mites that eat feather fragments and anything else organic that falls into the water and their predators, prismatic comb jellies. The jellies use a much weaker form of Prismatic Spray to stun and kill their food and constantly produce enough like to drive off the mites. If the rocs are slain or leave, the mites will migrate into deeper waters to find food and become a pest in the underearth. The jellies need sunlight to power their magic and so with either adapt to just sunlight or die off entirely.
The Red Marsh is an upwelling lake that only recently appeared. It drowned the lower parts of a city and with its waters came aberrant fish (Advanced Bestiary with giant gar and giant catfish as base creatures) and aberrant urchins (same template and AD&D Fiend Folio), the latter feeding on stone buildings as well as any vegetation they come across. The people have given up trying to drain the water as any attempts to drill new tunnels draws the attention of xorn that are happy to eat metal tools. And the spill off river drains down into a primeveal forest and has yet to connect with any other bodies of water. Still the fey and elves in the forest are not too happy about the arrival of wierd fish monsters.
Oonith Lake is set in the tooth of a giant space creature that fell to earth centuries ago. Not the creature, just the tooth. It pierced the ground and created the lake. It is populated by hybrids of displacer beasts and kuo-toa (Deluxe Book of Templates 3.5). How that happened no one knows, possibly a result of the alien nature of the tooth. In any case they are amphibious horrors that attack local towns at night and eat the people they capture or slay. What is worse is their mounts- hybrids of wyverns and tunnel worms (same template and AD&D Monster Manual II). These creatures can fly, swim, climb, and burrow so few creatures can escape them. That is why those town are only populated by the desperate or ego driven.
Singing Lake is found in a desert and surrounded by thousands of square miles of gravel. That is a good thing as the lake is populated by siren oozes (harpy song used by grey ooze) that lure living creatures (that can hear) to their doom. Normally a singing ooze's call only reaches a 100 feet or so but the lake's shape and number of oozes increases this to an astounding 200 miles, just far enough to reach into the closed settled lands. And at that range the effect is less like a harpy's call and more like Suggestion. The people occasionally set out to explore the desert and don't know why. The closer they get, the stronger the effect. At the lake itself, the song is overwhelming and even dragons succumb. Which is why there are few dragons native to this desert (be they blue, yellow, brown, brass, copper, red or any of the d20 desert species).
Glass Lake is another death trap, one located in the middle of a nation. It is populated by something carnivorous but what is unknown as the water renders all living flesh in it transparent. And the "invisible" monster somehow retains this trait when it comes out of the water to hunt. The locals keep this to a minimum by throwing criminals, livestock and strangers into the water. The actual creatures of the lake include some underearth fish, a giant snail that is much like the coneshell, a tribe of debased nixies being driven mad by their condition, and an amphibious orca (Advanced Bestiary and Monster Manual) that was accidentally created by a wizard in a distant land and was teleported away to get rid of it. It is quite cunning and has no difficulties with its transparency while hunting on land.