Why do we need to save water when we have a dam?

Nelson City’s water comes from three sources; the Roding River, the South Branch of the Maitai River and the Maitai Dam across the North Branch of the Maitai River.

Under normal conditions, the dam is used to supply some of Nelson’s drinking water as well as replace the water that is taken from the South Branch, and also supplement the river flows to maintain the health of the Maitai River.

The recent dry weather has seen all river flows drop as the city moves through stage one and two water restrictions, with stage three coming into effect at the end of last week.

Many people have asked why we need water restrictions when we have a dam.  

•  Under normal conditions the dam is self-sustaining.  The water that is fed into the dam from the North Branch is enough (along with water from the South Branch and the Roding River) to cover Nelson’s needs and allow the replenishment of the Maitai River to maintain required flows.
• In a drought, the river flows in the North and South branches of the Maitai River reduce and water from the Roding River becomes unavailable. 
• As the river flows drop, the amount of water entering the dam from the North Branch decreases, but we continue to supply water to the Tantragee water treatment plant and to top up the Maitai River to maintain flows. This causes the dam levels to fall.
• This means that in an extended drought like the one we are now experiencing our water supply is no longer self-sustaining. 
• At the same time, we experience increased demands on the water supply from our urban residential and commercial users, for irrigation, swimming pools and industrial uses. 
• As dam levels fall, the city water security is reduced and restrictions are put in place to ensure we do not run out of water. 

The dam provides good security of supply for the city but not an unlimited supply. As we do not know how long the drought will last Council adopts a cautious approach to water restrictions to ensure we have water for essential purposes. 

We enforce water restrictions to ensure that 

• Our water supply lasts until the next period of rainfall events
• The impact on the Maitai River catchment is within the bounds of our resource consent.