As you are talking about a hillside location, most natives will do well there, as the water should not be a problem. The Fremontodendron is the only one I can think of that might be a problem to get established. Al

Matilija poppies are lovely. They can take a lot of summer water. So can St. Catherine's Lace, Erigonum Giganteum. Very, very beautiful buckwheat plant. If there is any shade, Hummingbird sage is a pretty green and it likes water at least once a week in the summer. California Fuschia is a nice plant, and it attracts hummers as well.

Renee

Thanks for the helpful suggestions! I am new to growing natives and appreciate the plant list Renee.

I have a similar situation, my neighbors drip leaks onto our side of the hill. Like Renee says, St. Catherine's Lace is happy as well as my sugar bush (rhus ovata).

Bush Anemone (Carpenteria californica) likes water as well as monkey flowers & currants (if you have some shade for it).

Ick! I googled St. Catherine's Lace, and most of the pictures show horribly butchered plants. This plant gets huge, and it must be allowed to assume its natural shape, which is lovely.
Even this photo does not do it justice. It has silver foliage and creamy white lacecap flowers. The lacecaps are held horizontally, so you get that 'Japanese Maple' growth effect. Like a Japanese Maple, it should not be messed with or the shape is ruined.

Chezron, there should be more people chiming in with suggestions. Summer is slow on this forum.

Renee

Here is a link that might be useful: {{gwi:545133}}

Some of the Ceanothus do tolerate some summer water.
Check LasPilitas website below, it is a very valuable resource.
Here are a few other links also:
http://www.cnps.org/
http://www.gardeningwithnatives.com/#
http://www.theodorepayne.org/
http://www.mynativeplants.com/
http://plantnative.org/
http://nativeson.com/
http://www.wildscaping.com/index.htm
http://www.californiagardens.com/index.html
http://www.calown.com/

Here is a link that might be useful: LasPilitas

I would not plant any California natives before the winter rains start. Al

Although Al is technically correct (unless the summer rain or summer watering is happening during abnormally cool temps - it's the combo of heat and rain that do our CA natives in), many of the Ceanothus hybrids that have been developed recently do very well with summer water. I have Ceanothus Concha that does very well, even getting overhead sprinkled (no choice - it was here when I moved in and impossible to change the watering pattern or relocate). And the 4 Ceanothus I have that get regular summer overhead sprinkling are gigantic. It may, over their entire lifespan shorten it, and Ceanothus and not known for being long lived, but these are probably 7 years old or so, and show no signs of decline. Here's a link directly to Las Pilitas' Ceanothus hybrid web page. Las Pilitas is about 20 mins. from me, and they have an extremely well-documented web site. (And I respectfully disagree with Ceanothus being somewhat short-lived. Yes, our watering practices can hasten death, but many varieties are on the shorter-lived side according to David Fross, who probably the best source for Ceanothus alive.)
Timber Press: David Fross Bio

Patty S.

Here is a link that might be useful: Las Pilitas Nursery: Ceanothus Hybrids

Also if you are in a wildfire zone, check flammability before you plant.