During the events of Pokémon Diamond, Pearl, and Platinum, Team Galactic uses a Galactic Bomb to dry up the lake. The resulting earthquake is felt in Canalave City, located at the opposite side of Sinnoh. While dried-up, what appears to be the remains of a river that flows into the lake is located to the northwest of the lake. When the lake is restored, the remnants of the river disappear and are replaced by trees. While the lake is dried up, the cavern entrance is at the level of the lakebed, but while it is filled, the entrance is at the water level.
Ash and his friends arrived at Lake Valor for the Wallace Cup in A Full Course Tag Battle!. They were soon joined by May, who traveled all the way from Johto to compete for the Aqua Ribbon. Since the competition would not start until the next day, the group decided to visit the Seven Stars Restaurant, where they formed pairs to face off against the owners of the restaurant, Roman and Kylie, in Tag Battles, as it is required for customers to defeat them before sitting down to have a meal.
In The Needs of the Three!, it was explained that Lake Valor, Lake Verity, and Lake Acuity are the entrances to another world where the lake guardians reside, and the Pokémon do not actually live in the lakes themselves. In order to draw Azelf out from its homeworld, Pokémon HunterJ, having been hired by Team Galactic, dropped the Galactic Bomb at the lake, which created a spatial anomaly that repelled the water and forced Azelf to appear.
After that, J proceeded to capture Azelf, prompting Mesprit and Uxie to appear at the lake. J tried to capture them as well but was challenged by Gary, who was monitoring the lake just as Professor Rowan and Professor Carolina were doing at Lake Verity and Lake Acuity, respectively. However, Gary's Umbreon and Electivire were no match to Saturn's Toxicroak and were quickly defeated. After J successfully captured all three lake guardians and delivered them to Saturn, her airship was struck by Mesprit and Uxie's Future Sight, which the two had used before being captured. This caused J's ship to crash and sink into Lake Valor. Soon after sinking, the ship exploded, leaving the fates of J and her henchmen unknown.
At the end of Bombastic Bibarel & Heroic Hippopotas, Diamond, Pearl, Platinum, and Dr. Footstep were thrown to the Lake Valor lakeside by a sandstorm created by a wild Hippowdon. Diamond and Platinum ended up on the shore and while seeking for their comrades, Diamond told Platinum about a mysterious incident that happened to him when he was little at Lake Verity. After this, they encountered two Scientists, who intended to take them away, leading to a battle. Meanwhile, Pearl and Dr. Footstep had ended up on the opposite shore of the lake, but were thrown on an island in the middle of the lake by a wild Staraptor. In the midst of the battle between Diamond and Platinum and the Scientists, a strange light coming from the center of the lake gave Diamond and Platinum a chance to resolve the battle. Pearl and Dr. Footstep swam to the shore and saw the two Scientists tied up. After Diamond explained what had happened, Platinum fainted due to the burden of the battle. In the morning, she woke up, shocked upon noticing they had camped out the previous night, as opposed to her plan of staying at Hotel Grand Lake. After the first shock, she recovered and enjoyed the sunrise.
Later, after Diamond, Pearl, and Platinum decided to split up to stop Team Galactic, Pearl headed to Lake Valor. In Tackling Tangrowth, Team Galactic blew up the lake with a Galactic Bomb in order to awaken and capture the lake guardians. While being dragged into Valor Cavern by Saturn, Pearl revealed that he had seen a reflection of a blue-headed Pokémon on the surface of the puddle in the cavern while being there earlier. After Team Galactic had successfully captured Azelf, the dried-up lake was taken care of by Crasher Wake, who had his Quagsire and Floatzel use Rain Dance in order to refill it.
In the Pokémon Diamond and Pearl Adventure! manga
This section is incomplete. Please feel free to edit this section to add missing information and complete it.
Lake Valor dried up in Pokémon Diamond and Pearl Adventure!
The lake makes its first appearance in The Legendary Pokémon, Captured where it was all dried up from the explosion caused by Saturn putting off a Galactic Bomb.
When the bomb is set off here, the blast is powerful enough to be felt in Canalave City on the other side of Sinnoh, but there is no damage to the Magikarp living in the lake or the surrounding area, besides the obvious crater and disappearance of water.
Lake Valor is the only Sinnoh lake that is not entered from the south.
Valor means "courage". This is fitting, as Azelf is the Willpower Pokémon.
Risshi is from 立志 risshi, "resolve" or "aspiration".