Impossibly productive Futa!Dash taints Ponyville's water supply, replacing all the water and weather in Ponyville with her own cum.

Rainbow Dash glanced over her shoulder. Nopony was watching her, at least not from anywhere she could see. Good.

She smirked and plopped back on her haunches. The Ponyville reservoir rose up before her, its clear, filtered water glittering in the light of the noonday sun. All of the water here became clouds, became drinking water, became showers. It was one of the most vital - though unseen - parts of life in Ponyville.

And Dash was about to make it her own.

Dash bit her lips as her stallionhood emerged from its sheath, growing longer and longer until it stood at its full size of sixteen inches. It bobbed above her belly, the flared head inviting her to lean down, to give it a nice, moist, suck. But Rainbow Dash had more important things to do today, and it all rested with the two grapefruit-sized testicles sitting quietly between her legs.

She shifted her weight, chuckling as her cock slapped against her chest and her balls sloshed loudly. A bit of pre was already starting to drool from her tip. Looked like she was ripe and rarin’ to go. She took a moment to massage her balls gently, her enormous cock twitching and flaring beneath her chin as she felt the cum churning inside of her sack.

Finally, she was ready. She stood up on her hind legs - it wasn’t hard (though she certainly was); she’d always had a good sense of balance, and her wings helped to balance the weight of her sizable package - and pointed her member directly toward the reservoir.

One stroke of her hoof, and the whole thing throbbed, as if preparing itself for the flood soon to come. Another stroke, and her entire lower body tensed, flexed, tightened. She bit down on her lip, a bead of sweat forming on her forehead as she slowly started the third pump.

Another pony might have needed a few minutes, at least, to bring themselves to orgasm. For a pony as cum-filled, horny, and erect as Dash, though, the three pumps were more than enough.

Her hips bucked with her cock as the tension in her crotch reached a peek. The flare swelled even further as her balls pulled toward her body. A fat, throbbing bulge emerged from the base of her cock and moved upward, stretching her urethra almost unbearably wide with the volume of the ocean she was about to unleash.

With a cry, Dash flung back her head and thrust her hips forward. A veritable geyser of cum spewed forth from her tip, her balls swelling and pulsing with her cock as they pushed gallon after gallon of hot, steamy seed out of her tip. She felt like she was channeling a river of pure sex, so full were her balls and cock. And all of it was going into the reservoir.

Already, a puddle of thick, creamy white was gathering at the top of the water closest to her. Some of it sunk, and some of it floated, but all of her spunk clung together in a giant, growing cloud of virility. Dash groaned and grunted, the pressure in her cock and balls doubling with the rate at which cum flooded forth from her tip, more and more draining into the lake with each passing second. Her grapefruit-sized balls bobbed and sloshed between her legs, with the eruption from her dick as the only evidence of their ocean-like contents.

The white puddle spread as dozens of gallons turned to hundreds, Dash’s balls contracting and throbbing with pleasure every second that she came. Soon enough, a section of the lake that must have been twenty meters across was covered in a thick, creamy white. Birds resting in the water began to quack and squawk with distress as waves of Dash’s cum splashed against their wings, driving them to take flight.

More and more cum erupted from her cock. Her near-limitless balls weren’t beginning to feel even halfway empty, but nearly the entire surface of the reservoir was covered in her cum by now. As Dash’s orgasm went on and on, there was soon no more water to be seen - only an endless expanse of thick, creamy white that would soon be raining down from the skies, filling ponies’ cups, and washing their naked bodies.

Dash shuddered, biting her lips and closing her eyes as she continued to jerk off madly, trying to milk every second of her orgasm as much as she could. Soon, the entire lake was more than full of her cum. Soon, the waterline started to rise.

And still, Dash came.