The Aratiatia Dam loop as described here is the best ride to take along the Waikato River from town to Huka Falls and on to the Aratiatia dam and rapids. The trail loops down the right of the river, crosses it at the dam, and come back up on the left. It combines these trails as shown on the "biketaupo" map available from the i-site: Rotary Ride to Huka Falls, the Huka to Arariatia Trail, True Left Aratiatia, Contact Lookout, Wairakei Resort Trail, Totora Gully Trail, Redwoods, Rural Delivery, Acid Drip and back to Spa Park.
The trails vary from easy to advanced but the advanced sections can be avoided by going on the road. The trails are not great examples of singletrack artistry but they are interesting and take in some stunning sights. To see the mighty Artiatia rapids flowing you need to reach the dam when the flood gates are opened which is for just fifteen minutes every two hours from 10am till 4pm during summer. Allowing time for photos at Huka Falls it will take you about one and a half to reach the dam from the trail head.
Things to look out for along the way include Huka Lodge where the rich and famous stay, the Huka Falls - they are not high but they're impressive for the way the mighty Waikato is squeezed down a narrow channel, the SH1 overpass, the prawn farm, the hydro-thermal power station and steam pipes, the jet boat giving the innocent victims their adrenalin rush, the Aratiatia dam, the Aratiatia rapids, and the redwoods.