When it rains heavily the island's steep valleys and gorges turn into temporary rivers and its cliffs into huge waterfalls (caideros in Canarian Spanish). They only last for a couple of days so you have to hurry.
Once the rain stops, hire a car and head into the hills. Check local weather reports first as heavy rain sometimes blocks the roads and you don't want to be driving up there in heavy rain.
If the barrancos are running you'll see the water rushing downhill. Directly after rain it's a chocolate brown but it soon goes clear.
Here's where to go to see waterfalls:
Up the south valleys
If the Maspalomas barranco that ends up in the Charco lagoon by the lighthouse runs, then it's rained hard in the south of the island: Drive up the Arguineguín, Fataga or Mogan valleys and you'll see waterfalls on the cliffs all around you. The area around Tunte (San Bartolome de Tirajana) is a particularly good spot as you get a panoramic view of the cliffs.
The biggest waterfall by volume is just above the reservoir wall and the little village at Soria. Head up the Arguineguin Valley on the GC-505 until you get to Soria, then follow the dead-end road up to the car park. The waterfall is right there. At its peak, it's an enormous rush or white water.
There's another waterfall close to Soria village called the Cascada Bonita (Pretty Waterfall) or the caidero de Barranquillo Andres. It's just a short hike from the village and often lasts for weeks after heavy rain.
El Risco & Guayedra
The west coast of Gran Canaria is the steepest on the island and the best place to hunt for waterfalls. However, the west coast road is often closed during rain because of rockfalls so it's best to wait a couple of days after rain before visiting. Check with locals to find out if the west coast road is open.
If it's not, drive to Agaete via Las Palmas and head up the Agaete Valley for a close look at the water pouring off the Tamadaba massif.
Once the road opens you can drive to El Risco village and walk up the valley to Charco Azul and its blue waterfall. There's also a larger waterfall above Charco Azul but the path is difficult and we don't recommend it during wet weather.
The Guayedra Valley is just south of El Risco. Stop by the roadside when you get to the conical piles of canes and walk up the path past the reservoir until you get to the waterfalls. Don't stop at the first one as there are several in the valley.
Barranco de la Mina
One of the wettest spots on the island, Barranco de la Mina has a permanent stream and waterfalls, but they are at their best just after rain.
To hike up the barranco, park at Lagunetas village between San Mateo and Cruz de Tejeda and walk down the steps right by the church. Walk along the road at the bottom of the steps to the crossroads and turn left down the track into the barranco. You can also get to the Barranco de la Mina from Cruz de Tejeda.
Barranco de los Cernicalos
Another valley with permanent water, Los Cernicalos is more spectacular just after rain. To see its stream and waterfalls, drive through Telde to Lomo Magullo. The trail starts at the Los Arenales car park (see the gps spot on a map here) and is in good condition up to the two main waterfalls.
Charco de las Palomas
Pigeon Pool is a short walk from Tejeda village. The trail starts at a little square with a big sculpture of a basket. Just cross the road to find the trailhead and walk for a few hundred meters. Here it is on a map.