There are two excellent spots for swimming in the mountain stream next to Road 9甲 in Yuanshan. This is the perfect refuge during the hottest part of summer. The water is icy cold, and the surroundings are gorgeous.
Even though these places aren't marked, both spots are very popular. During summer weekeneds there will be quite a few people here. Even on summer weekdays therr will be some visitors,but it's a long stretch of river so you won't feel too crowded. If you're misanthropic, you can just wade upstream or downstream until you find an isolated spot.
Take Road 9甲 going west (toward the mountains) from the center of Yilan City. Keep following the road as it crosses the plain. After you pass the last house and start to ascend into the mountains, keep a sharp lookout for a little dirt trail on your right. You will come across it around one minute after you pass the last house and the road starts to rise up into the mountains. This trail isn't easy to spot, so you have to look for it. On the weekends there will be a couple of cars parked by the side of the road nearby.
Park and walk down the dirt path to the stream. The water here is cold and clean, and you have great scenery. Unfortunately, there isn't any shade here, so it can be uncomfortably hot on a summer day. On weekends people bring huge beach umbrellas and canopies and camp out for the day - they seem to spend most of their time barbecuing sausages. If it's a cloudy day or you bring an umbrella (or want to sunbathe), this is a nice spot. It's a great place to bring small children, as the water is very shallow and doesn't flow too fast.
To get to the second spot, keep driving up Road 9甲 for a few more minutes. You will see a little road that veers off to the right and crosses the stream over a small old concrete bridge. There is usually a vendor by the side of the road selling snacks, and there are always some cars parked on the road during the daytime in summer. Park and walk down to the stream.
This place is paradise. Just find a spot that suits your tastes and relax. You can soak in the cold water and enjoy the beautiful surroundings, full of frogs, dragon flies and butterflies, little fish and tadpoles. The pristine mountain water is bracingly cold. Even when it's 36 degrees down below, here you can hide in the shade and enjoy the mountain breeze. A dip in the icy water will make you forget how hot you were in Taipei.
If you bring a picnic lunch and some bottled tea or beer, you can easily spend all day here.