Due to the geographic features, the rivers in Sanyi can be divided into two water systems. One is the Sihu River water system from south to north, and the other is the Jingshan River water system from east to west. The upper streams of Sihu River include Shueiwei Stream, Chonghe Stream, Shuangtan Stream and so on, while Jingshan River is joined by Bogong Stream and its upper streams such as Sihfeng Stream and Longteng Stream. 

Sihu River:

Originated from the north foot of Guandao Mountain, it was originally called Danaba River. It runs towards northwest to join Shenshueikeng Stream around Changshou Bridge, and then continues northwest to join Shuangtan Stream at Shuangtaner Bridge. Chonghe Stream joins the Sihu River around Shuangtanyi Bridge. Its direction turns inwards around the Freeway, runs through Shuanghuyi Bridge and Sansing Bridge to be joined by Shueiwei Stream to form the mainstream. It then continues north to enter Tongluo Township. The total length of the Sihu River in Sanyi Township is 8.45 kilometers and covers 13.42 square kilometers as its catchment area. It is one of the secondary rivers in Taiwan. The water resource within its drainage area is quite abundant. However, due to the uneven precipitation, the water flow varies extremely at all times. In addition, there is no water reservoir to regulate the water flow. As the result, the water resource for efficient irrigation is limited, and it can mainly provide farm irrigation, some for industrial use and very little for public and domestic use. 

Shueiwei Stream:

Originally named Damu Stream, it originates at the north foot of Jitu Mountain around Shengsing. It runs north to pass by Singwu, through Sun Yet-sen Freeway and Sanyi downtown to arrive Sansing Bridge at the border of Shuanghu Village and Guangsheng Village and enters Sihu River. Its total length is 5.4 kilometers and covers an area of 10.48 square kilometers. It is in the category of ordinary rivers, and is also a youthful river. There is no obvious deep stream channel. The channel is narrow and stream flow is turbulent with high velocity. The water does not follow constant pathway and downward erosion predominates. 

Jingshan River (Liyu River):

It was originally called Duoluoyan River. Originated at the west of Malaban Range in Jhuolan Township, it then runs through Liyutan Village to enter Daan River around Huoyan Mountain. The project to built a multi-functional dam, namely the Liyutan Reservoir, at the middle and upper stream area started in 1986, and the construction work of the first phase was completed in 1992. 

Bogong Stream:

Originated at the Baguei Mountain area at the south of Neihu Village, the upper tributary is called Sihfen Stream. It runs south through the Sun Yet-Sun Freeway to enter Jingshan River at the north tip of Daan River Bridge. With a total length of 3.5 kilometers and catchment area of 7.5 square kilometers, it belongs to the category of wild rivers. The water route circulates the mountain. Its hinterland is quite limited and is only miscellaneous forest. The waterway is meandering, gradient is steep, and the velocity is high. It has great potential to be developed into tourism area. 

Longteng River:

Originated at the Longteng Mountain area, it flows into Jingshan River around the Guanyin Temple at Fenglong Mountain in Liyutan Village. Due to its steep gradient, there are some small waterfalls and big pools. The scenery along the whole stream is breathtakingly beautiful. The famous Wangcing Valley is situated at this river area. 

Liyutan Reservoir:

It is situated on the Jingshan River at the southeast of Liyutan Village. The government makes use of the special valley landform at the upper stream area of Liyu River, and started building the reservoir in August 1986. The construction work of the first phase was completed in June 1992. Apart from collecting water resource from Liyu River, the reservoir also includes an on-stream dam at the middle stream area of Daan River for river water collection. The water runs into the reservoir through conduits for storage and management. It is an off-stream reservoir, which is characterized by its extended life and water cleanliness. The main structure is a rolled earth and rockfill dam with 96 meters in height, 235 meters in length, and 20 meters in width. The reservoir watershed is 53.45 square kilometers with storage capacity of 126.12 million cubic meters. It covers Dahu and Jhuolan area, and is the biggest reservoir in the Miaoli County. With its functions of irrigation, flood control, electricity generation and public water supply, it can bear the aim to develop its natural landscape and resources into an overall tourism area.

Desing Pond:

Originally called Guaizai Lake, it is renamed as Desing Pond in 1959. Situated at the east of Nahu Village and close to West Lake Resort, it covers an area of 8 hectares. The beautiful valley and the greenish mountains reflected in the lake constitute a beautiful natural scenario. Due to the decreasing water storage capacity of recent years, the number of tourists is also going down and proper actions need to be taken to re-develop its tourism resources.