Special equipment: Kitchen string
In a medium saucepan, bring the water and rice to a boil over medium-high heat. Reduce the heat to medium and simmer, uncovered, for 18 minutes. Remove from the heat and drain excess liquid, if necessary. Transfer the rice to a medium bowl.
Melt the butter in heavy medium skillet over medium-low heat. Add the leeks and cook until softened, about 4 minutes. Add the mushrooms. Cover the skillet and cook, stirring occasionally, until the mushrooms release their juices, about 5 minutes. Uncover the skillet. Increase the heat to medium-high, and continue to cook until the liquid evaporates, about 3 minutes. Stir the mushroom mixture into the rice and mix well. Season with salt and pepper, to taste.
Preheat the oven to 375 degrees F. Season salmon fillets on both sides with salt and pepper, to taste. Working in batches, put 2 corn husks lengthwise in front of you with the wide side in front of you, overlapping each other on the long sides. Divide the rice into quarters. Put 1/4 of the rice in the center of the husks. Put 1 salmon fillet on top. Fold the long sides and ends over the rice and salmon filling. Tie with kitchen string. Continue with remaining rice, salmon, and corn husks. Arrange them on a baking sheet and bake until the salmon is fully cooked, about 25 minutes. Serve with dill sauce.
Dill Sauce: Combine the clam juice and wine in heavy small saucepan. Bring to a boil over high heat until reduced to 1/3 cup, about 5 minutes. Reduce the heat to medium and whisk in the crema. Simmer until reduced to 1 cup, about 5 minutes. Remove from the heat, then stir in the dill and cilantro. Season with salt and pepper, to taste.
The corn husks can be substituted with 6 by 8-inch pieces of parchment paper.