Linear perspective implies depth through the size and placement of the elements. Closer elements appear larger to the viewer. Vanishing points and horizon are features of linear perspective, though they may not be as noticeable in nature as with an environment that includes man-made elements.
Understanding and applying the principles of linear perspective will add depth through the structure of your drawing.
Assuming all of the trees are the same size, the trees closer to the viewer will appear bigger.
The parallel sides of the river visually converge in the distance, making or creating a vanishing point. The vanishing point in this scene rests on the horizon.
Though hidden behind trees and mountains, the vanishing point and horizon still influence the elements in this scene.
Another way that depth is expressed is through the overlapping of elements. In this example, the overlapping tree appears closer.