From the source, the water flows through channels into the terraced gardens of Tipon.
Built 600 years ago, it's an idyllic place.
300 people once lived here, explains a hydraulic engineer who's worked here for decades.
The ruin is situated three and a half thousand meters above sea level and is considered a
masterpiece of water management.
"Tipon was a huge ecological experiment, where the pre-Hispanic people made use of nature
and especially water.
It was used for rituals and ceremonies and also for agriculture.
Specific hydraulic systems were developed for these purposes - for example the channels,
the subterranean cisterns and the aquaducts."
The Incas succeeded in channelling the water over a course of several hundred meters down
the hillside, without it gaining pace, then sending it in various directions to irrigate
their gardens -- and then on down into the valley.
A few kilometers to the west is Cusco, the capital of the Inca Empire before the Spanish conquest.
Today over 2 million tourists visit each year.
The city is thriving, its population booming.
450,000 people live here.
But it hasn't lost its unique atmosphere.
The legacy of the Incas still survives.
Even the course of the river Sapi, outside the city limits, is a testament to Inca history.
Like in Tipon, the water is channelled into man-made basins to slow down the flow.
That also helps remove debris from the water.
But washed-up trash is the least of the problems.
Much worse is the raw sewage from thousands of toilets.
The stench is overwhelming!
This engineer works for the Sedacusco water utility company, which aims to improve the situation.
"Our goal is to decontaminate Cusco's water.
Including here in the river Sapi.
The water here is filthy and it's our duty to rid the river of toxins."
The problem is the many new housing developments on the outskirts of the city.
There are fifty in the northwest alone.
All generate sewage from toilets, sinks, baths, washing machines and dishwashers.
The river flows in underground channels to the neighbourhood of San Jeronimo
at the other end of the city.
From there it flows into a sewage treatment plant.
It's the pride of Cusco.
It's been undergoing renovation since 2014.
With the help of German consultants, it's being made more efficient.
"Working in Peru's most modern sewage treatment plant is very gratifying.
It's also very challenging.
Every day we investigate new technologies and learn about how cities in other countries
are implementing them -- things that are new to us in Peru."
Most of Peru's sewage ends up - unfiltered - in rivers, lakes and the ocean.
It's dangerously unhygienic and an environmental nightmare.
"Human waste generate approximately 75 kilos of carbon dioxide per year per person.
If it isn't treated, it could soon be 400.
This plant allows us to reduce human-generated emissions to 20 kilos of carbon dioxide per year.
You can smell the improvement.
The fermentation gas that accrues in sewage treatment plants is now filtered out more
efficiently - so the stench is far less overwhelming.
Much to the relief of the plant's neighbours.....
On Avenida Collana, for example.
This shop owner has been living here for seven years.
The street used to be notorious.
"When you came here by bus, the stop would be announced as 'Buenos Aires' - 'good air'.
That's what the neighbourhood was called.
Noone would visit.
Even the buses didn't want to come here."
It was especially bad at midday.
"In the past, when I would eat over there with my mother-in-law, we thought at first
that the stench came from the bathroom.
And anyone who visited us would notice the smell!
We'd always tell them that it came from the plant.
It's much better now.
It smells a bit.
But it's nowhere near as bad."
Delegations come from across the country to visit the sewage treatment plant -- on average
two a month.
The stench has largely disappeared thanks to the optimized biogas facility --
it prevents thousands of tonnes of greenhouse gases from being released into the atmosphere.
"We're in the process of deciding a framework for the biogas team.
The plan is for Cusco to convert 2000 cubic meters of gas into electricity by the end of the year."
The plant will soon be completely self-sufficient.
The water company will save over 20,000 euros in electricity costs per month.
The only question that remains is - how did the Incas deal with sewage?
"This is the bedroom.
The toilet, the bathroom, was probably adjacent."
"The bathroom?"
"We assume it was the bathroom on the basis of the floor plan and the ceramics we found.
Researchers have identified layers of organic material on pots, with layers of sand between them.
The Incas probably used them as chamber pots."
The cat litter principle.
Perhaps the Incas valued their water supply too highly to befoul it.