Normally, a river becomes wider the further downstream you go, as more tributaries add water to the stream. But Boulder's creeks become narrower as you travel downstream across the plains.
This is because our creeks are now part of an intensively managed water system. Upstream dams in the mountains store water for later use. Each successive irrigation ditch diverts more water from the creek and leaves less water in the creek. Cities divert water into treatment plants to supply homes and businesses. All this has drastically changed the character of the creeks around Boulder.
Source: Reflections in a Stock Pond, Robert Crifasi, Sept 2005