Final answer:
Rule #3 for riverine floodplain encroachment, while not explicitly stated, likely involves maintaining the ecological balance and not increasing flood risk through thoughtful development that respects the natural flow and services of the floodplain.
Step-by-step explanation:
Basic Rule #3 as it relates to encroachments in riverine floodplains is not explicitly mentioned within the provided references. However, based on the overview of the subject matter, which includes flood dynamics, ecosystem conservation, and the impact of development, it can be interpreted that key considerations should be taken into account when planning constructions near riverine floodplains. These may include understanding the natural flow of water, the effects on local ecosystems, and ensuring that development does not increase the risk of flood damage or affect natural water quality and levels adversely.
This understanding aligns with comments on preserving ecosystems to modulate water flow and mitigate floods, as well as the importance of proper management of water to prevent unintended consequences in agricultural areas. Therefore, an implied 'rule' might be ensuring that new development does not upset the ecological balance or increase flood risk, maintaining the integrity of the floodplain and the services it provides.