After the stream known as the Water flowed down from the northern hills of the Shire and through the village of Needlehole, its waters split into two. At this point in its course, the river slowed to form a wide bog, stretching for several miles either side of the Water itself. Little is known in detail of this marsh, except that wild rushes grew there in sufficient abundance to the give the bog its name.

Near the southern end of Rushock Bug, the two streams of the Water came back together, and it began its long journey southward and eastward. Some twenty miles to the southeast, it would reach Hobbiton, and then pass on eastward to eventually flow into the wide Brandywine. Along this latter part of its course, the Water split again to form yet another marshy region,² similar to that around Rushock Bog, but somewhat more extensive.

The map of the Shire within The Lord of the Rings shows a curious feature of the Water immediately after it flowed out of Rushock Bog. At that point the stream shown on the map seems to break briefly before resuming its course. The meaning of this break is unclear, though a similar break in the course of another river - Sirion in Beleriand - was used on a different map to indicate that the river ran underground for a brief stretch. Perhaps the same is true of the Water, or perhaps the broken line indicates another region of bog (which would effectively be a southward extension of Rushock Bog). Alternatively, the broken line of the Water may simply represent a slip of the artist's pen, now immortalised on the standard map of the Shire.