Swimming lake in Tokyo?

Im not a fan of the seawater, but I love to swim (yes, even this time of the year), so I was wondering if there is any nice lake/river/whatever without seawater in or near Tokyo?

I would prefer a non-crowded place.
A nice place to go with your girlfriend.

Ive been googling it for awhile but cant find any, and my japanese is still bad.
Please help me!

Thank you!

People don't swim in Japan in this temperature, so you are likely to get people calling the police if you ever swam in the river lol. You will be better off finding an indoor swimming pool facility in and around Tokyo. There is no decent lake in Tokyo.

I searched a lot for swimmable lakes, but there are very few places in Japan. Haven't found anything near Tokyo. There are a lot of lakes, mainly dams (forbidden), and even in some of the obvious swimming lakes like Towada-Ko in the North, the activity is deemed "kiken", dangerous, and is therefore not tolerated.

Here's some places on Honshu that allow swimming:
* Biwa-Ko near Kyoto
* Inawashiro-Ko in Fukushima-pref
* Suwa-Ko in Nagano-pref
* Ogawahara-Ko in Aomori-pref

In Hokkaido there are a few suitable lakes, and I hear Kyushu residents have many choices.

Any other Japanese lakes that we can swim in? Post the ideas please!! much appreciated.

Yukijin,

There are plenty of non-crowded, reasonable-priced, public swimming pools in Tokyo that are open all year round, but I assume you prefer cold water surrounded by nature.