Three Sisters of Plynlimon

Known as 'Tair Chwaer Pumlumon' in Welsh, the last word refers to a mountain and the giant sleeping beneath it both. Pumlumon, meaning 'Five Beacons', a mountain range in the Cambrian Mountains, Wales.

From this mountain are sourced three rivers, the daughters of Plynlimon: Hafren (the Severn), Gwy (the Wye), and Rheidol (Ystwyth). The three rivers are his daughters. As such, they are niskai (Celtic water nymphs/goddesses).

There are two tales of how the rivers were born. They aren't necessarily contradictory but they're never told together. The first focuses on how the sisters as sources were made whilst the second is how the rivers flowing from these sources came to be.

The Sisters

Despite the tales being short, these three sisters have massive, distinct peraonalities that remain consistent.

Severn (Hafren in Welsh) is the oldest, most beautiful and most powerful. She is also patient and slow. Despite this, she is quick to punish those who abuse her waters. 

Wye (Gwy in Welsh) is the middle child. She seeks beauty and harmony; she fills people with happiness and tranquility. The Wye is a creature of grace. 

Ystwyth (Rheidol in Welsh) is the youngest and smallest. She is headstrong nature and reaches decisions quickly. The Ystwyth skips and dances; she sparkles as she does so.

Version One: End the Dryness

Pumlumon wanted to bring water to his land in order for life to florish. He tried but failed everytime, leaving his land in a barren drought. 

Giving up, he shed three tears. These settled into three small puddles, from which were born his three daughters. 

Pumlumon nurtured these puddles to grow with both rain and mist. He in turn used his daughters' waters to nurture his lands, finally giving them life. 

When the sisters started to dry up, he encouraged them to go to the sea to save their lives. So they shot forth, created riverbeds to fill with freshwater. 
 

Version Two: Seek the River

The Three Sisters wanted to visit the sea and they discussed the best routes to the sea. They had different views, perfectly expressing their personalities. 

Whilst sad to see them go, their dad knew his daughters return riding clouds. From there, the three sisters would become the rain and mist to water the mountain and surrounding land.

The Severn wanted to visit as many human settlements as possible. She wanted to observe the humans, their celebrations and art. The Severn longed to gain knowledge and learn things.

The Wye took the scenic route. She was happy to wander without a specific path. The Wye can be found among the purple hills and golden valleys.

Ystwyth, on the other hand, was always in a hurry. She took the most direct route to the sea as possible, too headstrong to be deterred from her task.

How did the Rivers Fill with Water?

It's never explained how these three riverbeds were filled with water.
If the sisters were drying up as sources, they definitely wouldn't have enough water to fill entire rivers. 

The land itself was in a barren drought, dry enough for Pumlumon to seek solutions and constantly fail. This suggests there would be very little rain to fill the rivers up with. 

(But then rains and mists were necessary for growing his puddle-made daughters into sources large enough to bring moisture to the land. So it's unsatisfying to be 'just because'.)

Perhaps the rivers were filled up downstream and the water drove upwards to the sources like travelling an aquaduct? Perhaps magic? It seems impossible but I suppose that's what myths are for.

Conclusion

In story one, Pumlumon drove his daughters to go to the sea because they were drying up. In the second story, the Three Sisters just wanted to visit the sea. 

There are no reason why these can't coexist. Maybe Pumlumon wanting to save his daughters was the reason why the sisters wanted to visit the sea in the first place.  

(Even if they were incompatible stories, many things in folklore have multiple myths surrounding them. Why wouldn't the Three Sisters of Plynlimon be exempt from this?)

All-in-all, this is a story about fatherhood and sisterhood, about how vital life-giving waters are to us and all other life we share the world with.