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Yes, they bloom and depending on the species, a yellow flower. They make excellent food for Bees.
Here are some articles that have more information.

http://www.fcps.edu/islandcreekes/ecology/american_sycamore.htm
https://www.gardeningknowhow.com/ornamental/trees/sycamore/sycamore-tree-care.htm

I just learned about that. Is your tree a large one? Can you reach to spray each leaf, twig,and branch? If not you will have to get something injectable, Go to planetclinic.cornell.edu, there you will learn everything about this awful fungi and how to treat/save your sick sycamore as well as any others close by. Even our vegetable plants and fruit trees are susceptible once it's near them. Did you know that trees get canker sores too? Not only humans. Any way best of luck to you. Oh I almost forgot to share with you the most important thing. I used neem oil, and wow have my things bounced back. I didn't see anything about Neem oil for trees on the sight. I decided to give it a try since it did so well on my watermelon plants the bugs were eliminated over night then my plants doubled in size and began producing runners with the tendrils in 1 week maybe 10 days tops.I bought it at Walmart 9.99, will keep it on hand to water with every month or 6 weeks. just 2 tbls in a gallon of water. Well worth the investment. It is an citrus oil base which keep unwanted varmints away also. makes your tree strong also. Best of luck

Sycamores are not drought tolerant and it sounds like yours is being denied its typical water supply. It is possible for it to come back as long as the lake is refilled soon. Supplemental watering your sycamore until proper water level is restored is your best bet.

I am not sure what is wrong with the leaves, but it is probably either a disease or a nutrient deficiency. Anthracnose disease can kill leaves soon after they emerge in spring. A disease called canker stain (Ceratocystis) can cause small leaves and poor leaf development in sycamores. Zinc deficiency can also cause small and misshapen leaves. Consulting an arborist or extension agent may be the best option to find out what is wrong.

https://extension.psu.edu/pests/plant-diseases/all-fact-sheets/sycamore-diseases
http://www.clemson.edu/extension/hgic/pests/plant_pests/trees/hgic2011.html

Determining if there is an issue with a tree-due to slow leaf out can be a difficult task.
Cut into a bud-is it brown and dead inside or alive?
Test the branches; do they snap off and break or bend?
Did all the branches bud out?

Failure to leaf out can be due to a disease, root rot or pests.
Cold weather can also effect the buds.

I would remove a few samples of branches and take to your County Extension Office for inspection.

Here are some links for you including a link to help you locate your nearest office.

https://www.gardeningknowhow.com/ornamental/trees/tgen/tree-leafing-problems.htm
https://www.gardeningknowhow.com/ornamental/trees/sycamore/sycamore-tree-problems.htm
https://www.gardeningknowhow.com/extension-search/

Leaves dropping at this time of year are concerning.

Sycamore Trees are susceptible to anthracnose fungus.

The link below has more information for you.

http://www.clemson.edu/extension/hgic/pests/plant_pests/trees/hgic2011.html