| PACTEN | Conference before the additions of Colorado and Utah |
| GREENRIVER | Major stream of Wyoming, Colorado and Utah |
| MOUNTAINOUS | What Colorado and Utah are |
| PLUSSES | Additions of cats outside the pound (7) |
| ADS | YouTube video additions of 2007 |
| TOTALS | Last to get the additions (6) |
| RESPECT | Aretha Franklin song spelled out phonetically by the additions to this puzzle's theme answers |
| LEAP | Word associated with February 29 and the additions to 20-, 38-, and 57-Across |
| PLUS | A verbal rendering of the sign +, meaning "with the addition of", hence found in the name of baggy knickerbockers made with an extra four inches of fabric added to create an overhang at the knee (4) |
| UTAH | State that's west of Colorado and north of Arizona |
| SOPHOCLES | Greek tragic dramatist of the 5th century BC to whom such innovations as the addition of a third actor and the use of painted scenery are attributed (9) |
| ARAPAHO | Native American people who came from the plains of Colorado and Wyoming (7) |
| OTERO | County of Colorado and New Mexico |
| MECCA | City that's state capital of Colorado and California |
| ARAPAHOS | Native Americans of Colorado and Wyoming |
| PARAGOGE | In grammar, the addition of a sound, syllable or letter to the end of a word, as the st of amongst (8) |
| WREN | Royal Society co-founder who, following the Great Fire of London, redesigned St Paul's Cathedral including the addition of its dome, and a further 51 churches (4) |
| RIOGRANDE | North American river, rising in Colorado and flowing to the Gulf of Mexico, forming the border between the US and Mexico |
| PROSTHESIS | Experts' treatise on the addition of a sound or syllable at the beginning of a word (10) |
| DILUTION | The weakening by the addition of water (8) |
