Along the edges of the ponds of Delta Ponds, the willows are blooming with beautiful yellow blossoms called 'catkins'. I believe this is a Pacific Willow.

Here is a photo of catkins, just like those in my photo:
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More information about willow blooms:

"Willow trees' blooming phase begins in February in warm areas, and it continues until June in colder climates. The trees' long, tube-shaped flower clusters called catkins make their appearance just before leaves reappear on the branches. The flower clusters are filled with nectar, which insects carry for pollination. Within 45 to 60 days after pollination, willow seeds are ripe and ready to begin another reproduction cycle."

(See http://homeguides.sfgate.com/willows-bloom-50278.html)