The cows graze in parks shared with people. You don't want to fence around the people, just the cows. This is an example of a field that the cows visit regularly:
https://maps.app.goo.gl/1anrXAPRE3Dp3ZFo9
https://maps.app.goo.gl/YTfAYamG55SftWay6
https://maps.app.goo.gl/AWKxr76HUzgvSqBG7
People have free access to the river and the field, any physical barrier would limit that.