I learned that the pond is fed by a spring and it was larger in 1862.
Yes, the pond is spring fed; in the image in Post #8 the spring as at the rear of the left side of the pond.
However... there are myths about the Bloody Pond, such as it turning red on the anniversary of the battle or even if it was ever stained with blood at all. There is no contemporary evidence that the pond was stained red. In fact, there is no contemporary evidence there was even a pond there at the time of the battle.
The sole account comes from a civilian who walked by a pond several days after the battle seeing it stained with blood. But he did not make it clear on what part of the battlefield he found the pond. There were a number of maps made of the battlefield between 1862 and 1900 (Matz, Michler, Sneeden, C.F. Vent & Co., Buell's 1866 map for Century Magazine, Fermaux's map that accompanied Beauregard's report, and all of the Shiloh maps in the Atlas to Accompany the Official Records); none of them show a pond in that part of the field.
The pond does not appear until 1900 when Atwell Thompson's map was published. Thompson was the engineer of the battlefield when the Military Park was organised. He laid roads and constructed bridges and made an amazing map of the field to which the commission secretary Maj. D.W. Reed, added troop and camp positions. It was Major Reed, the father of interpretation of the battle, who wrote the first narrative history of the battle in book form. He also nailed up over a hundred signs in the spring of 1896 to help visitors locate the important parts of the battlefield, such as the Johnston Death Site, Shiloh Church, the Hornet's Nest, and the Sunken Road (which never was).
So why did Reed hang up signs for locations which had no historical background? It is very possible that he was asked where battlefield landmarks were that were actually features of other battlefields; Antietam and Frederickburg both had "Sunken Roads"; Chickamauga had a "Bloody Pond." Did he hang some of the signs as a joke? or did he just get tired of visitors asking for landmarks that were really in Georgia, Virginia and Maryland?
Whenever the pond actually formed, is has changed appearance over the years. The pond was part of a 43.50 acre tract that was purchased for the new park on January 8, 1898, from Mr. George H. Hurley. The photo with Maj. Reed's sign shows a fence right through the pond which actually encircled the pond but the photo was taken during the high water after a rain. A later superintendent plugged up the spring long enough to line the bottom of the pond with concrete. (The original cement pond??) Luckily it was later removed.
The Bloody Pond is an important landmark on the battlefield, a great help when orienting yourself on the field and to what troops fought nearby. It is just very doubtful it existed at the time of the April 6-7, 1862 Battle of Shiloh.