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I have a region in my homebrew D&D setting that used to be grassland/fertile farmland, and the breadbasket of the elven empire, until some wizards did a collective magical ritual that permanently altered the afterlife, made necromancy spells possible for arcane casters to use (they used to be solely the purview of the gods and their followers), and turned all of the casters into liches except one who became a death god.

As a side effect of that spell, I want the region that they cast the spell in to become a desert. Would it work to have it just be that lingering necromantic power is killing the plants for several decades? Would that be enough to turn fertile farmland into desert? (If not, I could also finagle an angry sun-goddess making things worse, but I'd only like to do that if her involvement is necessary to explain the transition.) What happens to grassland if the plants stop growing in it? I know cutting down trees can seriously ruin the soil of a rainforest, but does removing too much grass from a grassland have similar effects?

Also, the specific city they did the spell in was on the side of a lake. What would be plausible to happen to that lake? Would it dry up? Would it get clogged up by loose dirt being blown away in the wind? Or would it still be a lake, but just eerily lifeless now?

Ettina Kitten
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    Look into the great American Dustbowl. Also look into what happened to the Sahara. – elemtilas Jul 22 '20 at 14:06
  • This is a near-duplicate (a very near-duplicate) of Wastelands on a non-desertic climate: what would create them?. If that question doesn't answer your question, can you explain why not? Or are you simply asking if your scenario is feasible or realistic in your world-context (in which case you haven't provided enough information). – JBH Jul 22 '20 at 14:29
  • Also, please keep in mind that SE's basic model is one-specific-question/one-best-answer. SE isn't a discussion forum. We're lenient with new users, but for future reference, please focus your posts on a single question. You can find out more about how to ask questions (and what to avoid) in the [help]. – JBH Jul 22 '20 at 14:30

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So many questions! You may want to consider breaking this up in to multiple questions and rewording/thinking through them a bit more.

First off, a desert is defined strictly by the amount of precipitation. Antarctica is a desert despite having plenty of H20 around. It sounds like you want a barren, dry landscape devoid of all life? If so, then having a lingering magical spell which kills all plant life would probably accomplish this. No plants = no permanent animals, except in border areas (which in theory could be your entire area if small enough).

On to answering your title question: What would a lake look like if no plants were around. Then yes, there would be excess runoff (if you don't make it a desert and still allow rainfall). This would denude the landscape and wash silt and the like in to the surrounding bodies of water. Maybe you could leave bacteria and algae (maybe the magic doesn't work in the water on large plants) around to make the lake a putrid body of stagnant death if that fits your agenda?

Again, consider reworking your question. I can't make comments and I at least wanted to toss you a bone on where to start rethinking.

ColonelPanic
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For most purposes it would become a desert

Without plants holdingbthe soil, the fertile layers are washed away. Simply said, erosion. Without both plants and the fertile layers water isn't stored in the layers and the plants, so it'll find it's way deep into the ground, towards other bodies of water or evaporate quickly. This will make it a dry landscape. The ground will crack, allowing the process of water seeping away to happen more quickly. In the end, rain can be devastating as much as the drought, as it'll wash any beginning plantlife away.

At a lake I'm less sure. I expect bacteria that require low oxigen will start to thrive for a century or so, living on the remains that might still be around. Depending on how much of the dead matter is flowing away, they can live long. Large gummy sheets of bacteria would flow under the water, eventually going to the surface if it lost it's oxigen there, making for eerie stinking water. But eventually it'll die off as well when all plant matter and the bacteria are washed away, leaving only the shrunken or even dried up lake due to the hot sun. No plant life, no other life.

Trioxidane
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