It is not inherently illegal to collect rainwater on a federal level in the U.S. However, some states have restrictions or regulations on rainwater harvesting due to concerns about protecting the natural water cycle, water rights, and environmental impacts. The primary reasons for these laws include preventing disruption to water rights downstream, protecting soil health by allowing rainwater to seep into the ground, and managing water resources to avoid shortages. For example, in Colorado, rainwater collection was illegal for over 100 years because rainwater is considered public property that may legally belong to others downstream. Now, Colorado allows limited rainwater collection with specific restrictions like using sealable rain barrels and limiting the volume. In general, restrictions aim to ensure that rainwater collection does not interfere with water flows needed by neighbors or ecosystems, as well as to comply with longstanding water rights laws rooted in prior appropriation (first user rights). These rules often allow collecting rainwater from rooftops but limit collection from natural land surfaces to avoid depriving others of water runoff. Additionally, collected rainwater is typically restricted to outdoor non-potable uses due to contamination risks. In summary, rainwater collection is regulated to balance sustainability with existing water rights and environmental protection concerns, making it illegal or restricted in some places to prevent hydrological and legal conflicts.