Second attempt (first attempt below):
River delta
Line 1:
I am spread out like a leaf.
The water parts of a delta look similar to the veins of a leaf.
Line 2:
That what forms me cuts me, but builds me up.
Water erodes a delta, but also builds it up by depositing silt.
Line 3:
That what cuts me seeks always to go lower, but lifts its bed higher.
Water always flows downhill, but the "bed" of a river or river delta is built up by the sediment it carries.
Line 4:
I am commonly found where what builds me is less common.
The river delta is formed by fresh water, but the delta is at the ocean, where salt water reigns.
First attempt:
River OR riverbed
Line 1:
I am spread out like a leaf.
A leaf has "veins" that transport liquid, just like tributaries and smaller streams lead into the main river. If you looked at a system of rivers/riverbeds, it would look like leaf veins.
Line 2:
That what forms me cuts me, but builds me up.
Water forms a riverbed and "cuts" it through erosion, but also builds it up by depositing dirt, rocks, etc.
Line 3:
That what cuts me seeks always to go lower, but lifts its bed higher.
Water always flows downhill, but again the "bed" of the river is built up by the sediment it carries.
Line 4:
I am commonly found where what builds me is less common.
Not as sure about this one. If the answer is "riverbed" then those are often found where the water in the river has run out. Otherwise, it might be referring to the fact that rivers more often show up when there aren't lakes around (I'm not even sure if that is true). Still thinking on this line.