General aviation Flight Instructor reported a near miss with a helicopter while in the traffic pattern at a non-towered airport. The instructor took the controls and maneuvered away from the helicopter then landed safely.

Narrative

I was returning from a IFR training flight from ZZZ with my student. We we're VFR. We transitioned mid-field at PDX and then turned East when crossing the river northbound. From there we descended and entered on a left downwind for Runway 25 at 1W1. We made traffic calls on the CTAF when we were around 3 miles out and when over lacamas lake (a local landmark charted on the sectional and used by local air traffic). When we were over lacamas lake we saw a large black helicopter coming at us from our ~2 o'clock from the coming straight at us. My student was flying at the time but I saw it first so I took the control's. I decended quickly and turned left towards the airfield attempting to avoid it reaching an altitude of approximately 800msl (400agl) before feeling sufficiently separated from the helicopter. I attempted to raise the helicopter on XXX.X.(the local ctaf) but was unable to reach them. We landed safely. The area around lacamas lake is a choak point for inbound and outbound traffic from grove and also traffic inbound and outbound from TTD. Troutdale Tower will typically restrict traffic west of the lake but the helicopter appeared over the lake. I called Troutdale Tower after the event and they said that the helicopter reported me and sight and after that they authorized a frequency change to advisory. I could have likely paid more attention to traffic over the lake since it was a known choke point and thus seen the helicopter sooner.