What Lakes in Oregon are Prohibited to seaplanes by Mitch Swecker
US Army Corps of Engineers Oregon Lakes Seaplane Operations
 Waldo Lake closed to seaplanes

 Here you will find notes about places to stay away from.
Oregon Lakes Closed or with Cautions
Oregon State Marine BoardBe aware that there are new laws as defined by the Oregon Sate Marine Board,  prohibiting the use of some lakes. Here is a link to a web-page with
New boating laws in the State of Oregon including:

250-010-0650 Aquatic Invasive Species Prevention Permit

250-020-0240 No motorboat, except with an electric motor, on Cheadle Lake.
250-020-0221 Use of internal combustion motors in boats and floatplanes operating on the surface of Waldo Lake is prohibited year round.

See this list of all Oregon Lakes and their state regulations.

Lake Oswego - Private - Seaplanes not allowed.

Oregon Sate Marine and Aviation Regulations 
Keep a copy in your plane, you may need to educate a river patrol or ranger.

ORS 830.180 Use of motors prohibited on certain lakes

Except in an emergency, seaplanes shall not land, takeoff or operate on the following waters:
(1) Any body of water designated as a state or federal Wilderness or Primitive area or Wildlife refuge.
(2) Those waters listed in ORS 830.180 and in State Marine Board rules, OAR Chapter 250, Division 20, where motors are prohibited or that allow electric motors only. These restrictions are summarized and published in the "Oregon Boating Regulations" booklet available from the State Marine Board.
(3) Other bodies of water as designated by special regulations and adopted in OAR Chapter 738, Division 40.
(4) Those waters under federal jurisdiction that are closed to seaplane operations by federal regulations.
(5) Privately owned bodies of water without the permission of the owner.

State of Oregon Boating Access