Figure 1. This is a frame grab from a video
uploaded by
howarth004
in YouTube showing a great-looking formal koi pond
that has an informal waterfall in one corner. A
formal pond is one that has a definite geometric
shape (rectangular in this pond's case).
Figure 2. A close-up photo of the
'informal' waterfall. The term 'informal' means that
the waterfall has no specific geometric shape and
mimics the natural rock formation found in, well,
nature.
