Occurrence of Attributes in Original Text

The text related to the cultural heritage 'Mill Network at Kinderdijk-Elshout' has mentioned 'River' in the following places:
Occurrence Sentence Text Source
However, the drained soil continued to subside, while the level of the river rose due to the river's sand deposits.
It was decided to build a series of windmills, with a limited capacity to bridge water level differences, but just able to pump water into a reservoir at an intermediate level between the soil in the polder and the river.
The reservoir could be pumped out into the river by other windmills whenever the river level was low enough; the river level has both seasonal and tidal variations.