The Des Moines River serves as the marker for street names having the prefix "East" or not (and occasionally "West" when on the western side). This is especially important for streets running north-south. "1st Ave" would be on the west side of the river, E 1st Ave the eastern side and are thus completely different streets. Streets running east-west and which exist on both sides of the river typically have an "East" prefix if east of the river. There are some "West" prefixes for those on the other side of the river but this is less common. Perhaps more confusing, there are streets which lie on both sides of the river, such as Grand Ave, which is "E Grand Ave" east of the river but simply "Grand Ave" west of the river; yet "E University Ave" is officially named "W University Ave" when on the western side of the river. Unlike the north-south streets, it is possible to arrive at your destination by simply continuing east or west—depending on your final destination. The demarcation for "SE" and "SW" prefixes is a bit more ambiguous but would certainly refer to a street south of Grand Ave/E Grand Ave and either the western or eastern side of the river. For most part, "NW" refers to streets west of the river and "NE" east of it, but not always so. Additionally, there is no standard reference point for determining when a street gains a "NW" or "NE" prefix.
