The rocks represent the skeleton of the earth, its strength, the Yang . If mountains are the skeleton, then streams, rivers and lakes are the arteries and body, the Yin . Water should be in a natural state for the typical garden as a waterfall, stream, or pond. Fountains were only for entertainment used in the Imperial or wealthy person's gardens. Water is to be flowing over rock or a wall naturally, or still in a pond reflecting the strength of the rock.
Water replaces the grass or lawn seen in many European and American gardens.
To the Chinese a grassy field would represent a grazing area for cattle, not a place of contemplation. Making a garden in China usually means piling or arranging rock, and digging ponds. These are essential. Plants are a third element. Trees, shrubs, flowers, and herbs are selected for their beauty, mystical representation, and form.