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Hello and thank you for using this site to help you. I see that you are offline at this time. Reasons for scratching absent the presence of fleas can be dry skin in which case adding some coconut oil to the daily dinner will help and I will direct you to a site for dosing of this, and another reason which is fairly common would be airborne allergies which usually come with a season change.
Of course since she was swimming in lakes you want to be sure she did not pick up any skin parasites that you cannot see. Your Vet would need to do a skin scraping to determine this and likely you would see hair loss as well.
If the scratching/itching does not go away you will want to talk to your Vet about medications for possible allergies.
coconut oil dosing, warm it up in the microwave and pour it over the food.
seasonal allergies http://www.petplace.com/article/dogs/diseases-conditions-of-dogs/features/seasonal-allergies-and-your-pet