You can, but the first thing you should do is see where it comes from and, equally importantly, where it goes to.
You need to check what it does outside your garden.

Yes, you could wind up with a polluted pond, or drying up someone elses water feature, or a wildlife watering hole.

I agree with both the above comments and would add to them. You say it is only a trickle in summer - what about in winter? How will your proposed pond affect the way the water flows both before it reaches your garden and after it leaves your garden. Mr MG's parents had a stream flowing through their garden, it was in a very deep channel because when it rained hard the water off the hillside rapidly turned it into a raging torrent. A neighbour downstream decided to raise the level of the stream bed in his garden so that he could see the water all year round. Result the gardens, and houses, above him flooded in very wet weather. It took years to discover he had done this during that time Norah & Gerald suffered several floods and the council tore their hair out trying to discover what was causing the problem. So make absolutely sure you will not be affecting anyone or anything else before you make your pond.

perhaps take advice from the river authority people - perhaps omebody else knows their name.

Thanks for your replies. Just to reassure you that it really is little more than a trickle and this is not a wet area so not much more in rainy weather. It is inspected annually by our local water dept and it disappears underground and goes under our lawn, eventually meeting a local river. I don't want to dam it, only widen it a little to form a small pond, the water will continue to pass through the garden and wondered what plants would thrive in semi-shady conditions in it or round it.