The First Letter: Under this system, there is an alpha code for each of Harris County’s watersheds, beginning with “A” for Clear Creek, in the extreme southeast part of the county next to Galveston Bay. Moving in a roughly counter-clockwise direction, “B” is for Armand Bayou, “C” for Sims Bayou, “D” for Brays Bayou, and so on. Two notable exceptions: Little Cypress Creek, which is actually a tributary of Cypress Creek (“K”), nonetheless has its own alpha letter (“L”). And, while there is a “U” for the Addicks Reservoir watershed and a “W” for the Buffalo Bayou watershed, there is no watershed assigned to the letter “V”.
The First Number: Following the watershed letter is a sequence of seven numbers, grouped and separated by dashes. The very first number is important, because it tells what KIND of infrastructure the Unit Number identifies – “1” for a bayou or other channel, “5” for a stormwater detention basin, etc.
A watershed is like a tree, with one trunk (or main channel), multiple branches (tributaries) and twigs off each branch (sub-tributaries and sub-sub-tributaries). Each channel, no matter how large or small, has a unique Unit Number.
Let’s look more closely at Unit Numbers beginning with the numeral “1,” which denotes a bayou or other channel:
The first set of three numbers after the watershed letter in a particular Unit Number refers to the main channel or other prominent branch on which the channel in question is located. W100-00-00 is the Unit Number for Buffalo Bayou, the main channel of the Buffalo Bayou watershed. The main channel always has a “1” and lots of zeros!
W140-00-00 is the Unit Number for a major Buffalo Bayou tributary commonly known as Spring Branch. Other examples of Buffalo Bayou tributaries include W156-00-00 (Rummel Creek), and W167-00-00 (Turkey Creek).
When other than zeroes, the second and third sets of numbers in the Unit Number indicate smaller sub-tributaries, and even smaller sub-sub-tributaries, of the main channel. For example:
• W140-00-00 is a tributary of Buffalo Bayou (W100-00-00) known as Spring Branch;
• W140-06-00 is a tributary of Spring Branch known as Buttermilk Creek; and
• W140-06-01 is a tributary of Buttermilk Creek (with no common name).
In the beginning, Unit Numbers reflected a tributary’s location – in order – from the mouth or confluence of the main channel, working upstream. (For example, tributary W133-00-00 is upstream of W130-00-00 – or you could say that W130 is closer to the mouth.) However, manmade tributaries were later added to the system and given the next available number.
Stormwater detention basins: In the case of stormwater detention basins, which are identified by the first numeral “5” after the watershed letter, subsequent numerals help identify the tributary into which the basin discharges. For example: Basin A520-03-00 is the third of several basins that discharge into tributary A120-00-00.
Unit Numbers are important because they are unique, and help positively identify a particular channel. Common names are popular, but they are sometimes in popular dispute, or repeated in multiple locations across the county. For example: Harris County has three Turkey Creeks, in the Cypress Creek, Clear Creek and Buffalo Bayou watersheds. Only Unit Numbers tell you definitively which channel is which, and make sure everyone is talking about the same one!
IMPORTANT NOTE: The Flood Control District assigns Unit Numbers to channels in Harris County for informational and record-keeping purposes, including many for which it does NOT have right-of-way, or maintenance responsibility.