The image below shows the four Rainwater families in Spartanburg County, South Carolina in the 1820 US Census. Celia and Gabriel Rainwater would have been in one of those families. 

I have only Mazie's Family Tree (drawn by Rosemary) to show that Elizabeth Cannon was a daughter of Ellis Cannon and Barbara Huffman (or Elizabeth Coleman). I cannot be sure that she was born in 1770 in Overwharton Parish as I do not yet  find any source for that information from another researcher.

• Levi Rainwater — marked in green -- appears to be a father of three boys and six girls with a wife who is between 26 and 45 years old. They have no slaves. There are two of them working in manufatures, not in farming.To fit here, Elizabeth would have been born after 1775 and before 1794.
• John Rainwater — marked in red --  has no children. He and his implied wife are both over 50 years old. He works in agriculture and has 5 slaves. If Elizabeth Cannon was born in 1770 and married a Rainwater, then this is likely her household. It is very unlikely to be where Celia and Gabriel were born.
• Burrel Rainwater — marked in yellow -- appears to be a young father of four daughters and one son. He appears to be between 26 and 45 years old and his likely wife is less than 26 years old. If he married Elizabeth Cannon, then she would have been born after 1792. Ellis would have been older than 46 when she was born. He works in agriculture and owns no slaves. 
• William Rainwater — marked in blue -- Also appears to be a young father of 2 sons and a daughter, all under the age of 10. He and his wife are both between 26 and 45 years old. He works in agriculture and owns no slaves. To fit here, Elizabeth would have been born after 1775 and before 1794.

I have not found birth records for all of Ellis Cannon's children yet. So I will continue to look for Virginia church records in Stafford and Culpepper counties and in Spartanburg South Carolina. In addition, I will look for ways to eliminate each of these Rainwater households and to locate other family members for Celia and Gabriel Rainwater, along with their birthdates for comparison in these columns.

Based on all three US Census records for 1850 to 1870, Celia was born in 1799 or 1800 and would have been 20 or 21 on the 1820 US Census, if she was recorded correctly. She could be in either Levi's household with a younger brother aged 15 and a sister close to her age and one older female who is likely to be Levi's wife. She is less likely to be the oldest female in Burrel Rainwater's household, because that looks to be his young wife and Celia marries Elijah Cannon ( a fact that is proven by sufficient documentation).

Updated with new text and images on 2019 Feb 24.

In the next instance, Celia Rainwater is the wife of Elijah Cannon, who is a son of Nancy Wyatt and a grandson of Ellis. In this branch, Celia is the mother of Charity, Elhannah, Enoch, and Ellis. In both instances, Celia Rainwater is in the same generation at two branches removed from Ellis Cannon at the trunk. Is there any documentation that connects these two instances as either related individuals or as the same person?

If it turns out that there is only one Celia Rainwater in this tree, then she and her husband Elijah had the same grandparent, Ellis Cannon. That would make them first cousins of course and this was not uncommon in their time. I suspect it is true but I will need a lot more evidence before I can make a reasoned argument. As a soft conjecture I see that Elijah and Celia started their family in South Carolina close to the time that Mazie's great grandfather Nahum traveled to Tennessee to marry his first cousin, Cynthia Wyatt. Their children grew up far apart in South Carolina and Arkansas, yet Mazie's father Ed Cannon was already 14 years old when his great uncle, Elijah died. If he did not know them personally, he likely had a few stories from his South Carolina cousins, perhaps enough to have known that his great aunt and uncle were also first cousins. Again it is wildly speculative until there are some records to connect Elijah's wife Celia to Gabriel and to common parents. 

I do not have a full name for the spouse of Elizabeth Cannon Rainwater. ​We have only their father's surname and not his first name.  Next I will be continuing to reach out to my Ancestry contact who also has Celia and Gabriel Rainwater. Together I hope we can find the documentation that proves that our Celia and Gabriel Rainwater match up neatly.