1. aquifers
a small fertile area in the desert, usually a place where water is close to the surface (plural - oases)
2. delta
underground water sources
3. oasis
a broad, triangular-shaped land area at the mouth of a river
4. rain shadow
an area that receives less precipitation because of its location on the leeward side of a mountain

I'll assume you have to match the name with their definitions.

1. aquifers

underground water sources  

That's because of the presence of porous rocks that let water pass through them and reach an underground cavern that then acts as a tank.

2. delta  

a broad, triangular-shaped land area at the mouth of a river

That's common especially in larger rivers that meet their destination point (usually the ocean).

3. oasis  

a small fertile area in the desert, usually a place where water is close to the surface (plural - oases)  

We all heard about oases in the desert, where human and animals alike gather to drink.

4. rain shadow  

an area that receives less precipitation because of its location on the leeward side of a mountain

That's common in high mountains region where the mountains are so high they block the passage of clouds to the other side.