Hillside springs occur on a hillside where the water table meets an impermeable layer.

Here's why:

* Water Table: This is the upper surface of the zone of saturation in the ground, where all the pore spaces are filled with water.

* Impermeable Layer: This is a layer of rock or soil that does not allow water to pass through it easily (like clay, bedrock, or compacted soil).

When the water table intersects an impermeable layer, the water has nowhere to go but upwards, creating a spring at the point where it emerges from the ground.