The code letters in the key below each indicate a particular purpose designated for each dam built; combinations of letters indicate combinations of purposes, since dams can have multiple designated purposes. A table of the purpose code follows the chart, below.
| Dam Purpose Code | Purpose |
| I | Irrigation |
| H | Hydroelectric |
| C | Flood Control and Storm Water Management |
| N | Navigation |
| S | Water Supply |
| R | Recreation |
| P | Fire Protection, Stock or Small Farm Pond |
| F | Fish and Wildlife Pond |
| D | Debris Control |
| T | Tailings |
| O | Other |