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I am imagining a temperate forested area with freshwater lakes, rivers, small mountains, and rolling grassy hills. This environment is situated within a very large explosion crater. Outside this crater is desert for miles in every direction. Under what conditions could such an environment develop and how would it be maintained (i.e., stopping the desert from "taking over")?

Assume that there is no artificial environment/dome within the crater area.

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    $\begingroup$ Certainly. Except for the crater, you can find such places in e.g. the Great Basin of western North America. For instance the Stillwater NWR fws.gov/refuge/stillwater which is a wetland surrounded by desert. For your crater, you'd need a rim high enough to collect a snowpack. $\endgroup$ – jamesqf Nov 13 '16 at 18:43
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It's probable given the right circumstances

The most important thing here is water. Now there's actually quite a lot of water in most deserts, but it's in general hidden beneath the ground. Thus assuming your meteorite/explosion crater had a big enough impact, there's a fair chance that there's now a lake in the center of that crater, fed by underground reserves even.

Lakes? > check (well at least a big one in the center)

Now that we've got water we need to ensure it does not just evaporate and get carried away from our crater-oasis. Assuming a big enough impact we should have walls/sides tall enough to force at least part of the evaporated water to fall in the form of rain again. Thus we get a distribution of the water over a bigger area as needed (alas unless it's been a truly tremendous impact, some of the water will be lost and carried away permanently..)

Rivers? -> probably (there will at least be streams and brooks)

Now the mountains and hills thing is something else. As this is a crater made by an explosion, it will resemble a huge bowl most likely. So tall hills and little mountains will be restricted to the rim of it, alas these walls could be as tall as you manage to get them, so there's your mountains.
Additional thoughts thanks to @Zxyrra: As we've got a weather-cycle going, we can add more lakes & ravines coming from the crater sides towards the central recess; thanks to erosion.

Mountains? Hills? > well, we have crater walls. That counts for something, right? sure!, just make the whole thing old enough.

So there you go. Having a forest & grasslands inside a huge crater is definitely probable; Though the real question is how you got that crater in the first place?. You know, without blowing the planet up or at least destroying all life on it.

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    $\begingroup$ This is a thoughtful answer and I can't do better, but here's some information that may add: mountains and hills inside the crater could be explained by erosion as water moves down the crater's slope. Think "Grand Canyon" on a not-so-grand scale, and think "North American" desert instead of sandy Sahara. $\endgroup$ – Zxyrra Nov 13 '16 at 22:22
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    $\begingroup$ "You know, without blowing the planet up or at least destroying all life on it" - that might explain that desert outside ;) $\endgroup$ – PTwr Nov 14 '16 at 13:48
  • $\begingroup$ What a fantastic answer! Thank you - this gives me enough information to adjust my world's history to make this realistic. One question though - Is there any possibility of the desert reclaiming that area? Or will it more or less remain that way so long as average temperature in both the desert and the crater does not change? $\endgroup$ – syntonicC Nov 15 '16 at 15:55
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There is also a fairly common effect where you get a hill/mountain in the centre of the crater lake, because the ground "splashes" up after the impact. (This is Manicoagan Reservoir in Canada. You can see a little of the crater rim in the top right corner.)

Manicuagan

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