So what was the inspiration for the Four Rivers fountain? We can determine that from the unpublished sketch. The artist used the Tortoise Fountain as his source of inspiration. Compare the drawing to the Tortoise Fountain and there can be no doubt. Jacopo della Porta relied heavily on sculpture for his Tortoise Fountain, in contrast to his normal architectural fountains. Bernini is able to achieve a better proportion between the sculpture and architectures; he also creates an interesting round creation of rocks in comparison to the cold non-existent architecture of the Tortoise Fountain. Bernini also gives his figures a task, where as della Porta has no justifiable reasoning for his statues. Finally Bernini is able to make a more lively and energetic fountain by enlarging the dolphins and using vigorous jets of water, instead of the squashed dolphin’s seen in the Tortoise fountain.